Life Cycle
Even though I love playing Stardew Valley, sometimes I feel like I'm the only one progressing in town. This made me decide to make a mod that allows NPCs to date each other and much more.

Installation:
Each couple has a config.json located in Life Cycle - {Pairing name}/[CP] Life Cycle - {Pairing name}. You'll be able to change the fields 'Version' (allowed values 'dating', 'married'). Please, check first what versions are available for the pairing you're installing.

I recommend downloading the mod Generic Mod Config Menu to switch between versions more easily.

Dating version:

SMAPI and Content Patcher must be installed.
Download the main file and unzip it.
Delete from the folder the pairings you do not want (keep the Wedding Arch folder) and install only one pairing per NPC.
Place the main folder Life Cycle in your Mods folder.

If you're starting a new save and are installing Marnie-Lewis, change the variable for 'Save' from 'old' to 'new'.

If you're installing Emily-Clint and you have seen Emily's 8 hearts event and therefore can't see Clint's 6 hearts event, change the variable for 'Save' from 'old' to 'new'.

Married version:

SMAPI, Content Patcher and Mail Framework Mod must be installed.
Make sure you have the folder Life Cycle - Wedding Arch.
Complete all dating events for the couple you're installing.
Open the config.json and change the variable for 'Version' from 'dating' to 'married'.

Features:

New heart events (canon pairings are all finally done :) ).
New dialogue lines after every event for the couple.
After the 8 heart event is triggered, the couple's schedules change and they will occasionally meet up.
Also, after the 8 heart event, new dialogue lines are added to their family, that depends on your heart level with them.
Optional wedding event.

Available pairings:
Dating version:

Penny and Sam
Maru and Harvey
Emily and Clint
Marnie and Lewis
Haley and Leah
Abigail and Sebastian
Alex and Haley
Elliott and Leah
Emily and Shane

Married version:

Penny and Sam
Maru and Harvey
Haley and Leah
Abigail and Sebastian

Heart events info:
Penny and Sam

2 hearts: You must have 2 hearts with Penny and Sam. Enter the Museum on Tuesday, Wednesday or Friday between 10 and 2 o'clock.
4 hearts: You must have 4 hearts with Penny and Sam and have seen the 2 hearts event. Enter Sam's house between 1 and 4 pm.
6 hearts: You must have 6 hearts with Penny and Sam and have seen the 4 hearts event. Enter the town between 10 and 12:30 am. It must be sunny and not be winter. This event won't show up if you are married to Sam or Penny.
8 hearts: You must have 8 hearts with Penny and Sam and have seen the 6 hearts event. Enter the beach between 8 and 11 pm on a sunny day. This event won't show up if you are married to Sam or Penny.
Wedding: You must have seen all heart events. On spring 22 of year 3 or later, you will receive a wedding invitation (if sunny). You won't receive the mail if you're dating or married to Penny or Sam. After receiving the letter, enter the town between 4 and 5:30 pm.

Maru and Harvey

2 hearts: You must have 2 hearts with Maru and Harvey. Enter Harvey's room between 10 and 12 o'clock.
4 hearts: You must have 4 hearts with Maru and Harvey and have seen the 2 hearts event. Enter the museum on Saturday between 10 and 3 o'clock.
6 hearts: You must have 6 hearts with Maru and Harvey and have seen the 4 hearts event. Enter the mountain between 8 and 11 pm. It must be sunny and not be winter. This event won't show up if you are married to Maru or Harvey.
Special 7 hearts: You must have 7 hearts with Harvey and have seen the 6 hearts event. Enter Maru's house between 4 and 7 pm. This event won't show up if you are married to Maru or Harvey.
8 hearts: You must have 8 hearts with Maru and Harvey and have seen the 7 hearts event. Enter the town between 4 and 6 pm on a sunny day and not winter. This event won't show up if you are married to Maru or Harvey.
Wedding: You must have seen all heart events. On fall 7 of year three or later, you will receive a wedding invitation. You won't receive the mail if you're dating or married to Maru or Harvey. After receiving the letter, enter the town between 4 and 5:30 pm.

Emily and Clint

6 hearts: You must have 6 hearts with Emily and Clint and have seen Clint's 6 hearts event. Enter the railroad between 7 and 9 pm. It must be sunny and not Friday. If you change the field 'Save' to 'old', it's not necessary to see Clint's 6 hearts event.
7 hearts: You must have 7 hearts with Emily and Clint and have seen the 6 hearts event. Enter Emily's house between 7 and 8 pm. This event won't show up if you are married to Emily.
8 hearts: You must have 8 hearts with Emily and 10 hearts with Clint and have seen the 7 hearts event and Emily's 8 hearts event. Enter the Secret Woods between 3 and 5 pm on a sunny day. This event won't show up if you are married to Emily.

Marnie and Lewis

4 hearts: You must have 4 hearts with Marnie and Lewis. Enter the town on a sunny day between 9 pm and 12 am. If you are playing with a new save, make sure to trigger this event before you reach 6 hearts with Marnie and Lewis. Otherwise, the original event will be played.
6 hearts: You must have 6 hearts with Marnie and Lewis and have seen the 4 hearts event. Enter Marnie's ranch between 4 and 6 pm.
8 hearts: You must have 8 hearts with Marnie and Lewis, 6 hearts with Shane, have seen Shane's 6 hearts event and have seen the 6 hearts event. Enter the saloon on Friday between 7 and 9 pm.
10 hearts: You must have 10 hearts with Marnie and Lewis and have seen the 8 hearts event. Enter the forest on a sunny day between 6 and 7 pm.

Haley and Leah

2 hearts: You must have 2 hearts with Haley and Leah. Enter the forest on a sunny day between 11 and 13 o'clock.
4 hearts: You must have 4 hearts with Haley and Leah and have seen the 2 hearts event. Enter Leah's house between 4 and 8 pm. This event won't show up if you are married to Haley or Leah.
6 hearts: You must have 6 hearts with Haley and Leah and have seen the 4 hearts event. Enter the beach on a sunny summer day between 5 and 7 pm. This event won't show up if you are married to Haley or Leah.
8 hearts: You must have 8 hearts with Haley and Leah and have seen the 6 hearts event. Enter the forest between 12 and 2 pm on a sunny day and not winter. This event won't show up if you are married to Haley or Leah.
Wedding: You must have seen all heart events. On summer 27 of year three or later, you will receive a wedding invitation (if sunny). You won't receive the mail if you're dating or married to Haley or Leah. After receiving the letter, enter the town between 4 and 5:30 pm.

Abigail and Sebastian

2 hearts: You must have 2 hearts with Abigail and Sebastian. Enter the town between 11 and 12 o'clock.
4 hearts: You must have 4 hearts with Abigail and Sebastian and have seen the 2 hearts event. Enter the mountain on the weekend between 6 and 8 pm. It must be sunny and summer.
6 hearts: You must have 6 hearts with Abigail and Sebastian and have seen the 4 hearts event. Enter the forest on a sunny Sunday between 4 and 7 pm. This event won't show up if you are married to Abigail or Sebastian.
8 hearts: You must have 8 hearts with Abigail and Sebastian and have seen the 6 hearts event. Enter the mountain on a sunny day between 8 and 9 pm. This event won't show up if you are married to Abigail or Sebastian.
Wedding: You must have seen all heart events. On winter 12 of year three or later, you will receive a wedding invitation. You won't receive the mail if you're dating or married to Abigail or Sebastian. After receiving the letter, enter the town between 4 and 5:30 pm.

Alex and Haley

2 hearts: You must have 2 hearts with Alex and Haley. Enter the town on a sunny day between 5 and 6 pm.
4 hearts: You must have 4 hearts with Alex and Haley and have seen the 2 hearts event. Enter the bus stop on Friday or Saturday between 6 and 7 pm.
6 hearts: You must have 6 hearts with Alex and Haley and have seen the 4 hearts event. Enter Alex's house between 7 and 8 pm. This event won't show up if you are married to Haley
8 hearts: You must have 8 hearts with Alex and Haley and have seen the 6 hearts event. Enter the beach on a sunny summer day between 11 and 1 o'clock. This event won't show up if you are married to Alex or Haley.

Elliott and Leah

2 hearts: You must have 2 hearts with Elliott and Leah. Enter the saloon on Friday on Sunday between 5 and 7 pm.
4 hearts: You must have 4 hearts with Elliott and Leah and have seen the 2 hearts event. Enter the beach on a sunny summer day between 1 and 3 pm.
6 hearts: You must have 6 hearts with Elliott and Leah and have seen the 4 hearts event. Enter Leah's house between 1 and 2 pm. This event won't show up if you are married to Elliott or Leah.
8 hearts: You must have 8 hearts with Elliott and Leah and have seen the 6 hearts event. Enter the beach on a sunny day between 8 and 9 pm. This event won't show up if you are married to Elliott or Leah.

Emily and Shane

2 hearts: You must have 2 hearts with Emily and Shane. Enter the saloon on any day except Sunday between 6 and 8 pm.
4 hearts: You must have 4 hearts with Emily and Shane, have seen the 2 hearts event and Shane's 4 hearts event. Enter the saloon on any day except Sunday between 5 and 6 pm.
6 hearts: You must have 6 hearts with Emily and Shane, have seen the 4 hearts event and Shane's 6 hearts event. Enter Emily's house between 12 and 1 pm.
8 hearts: You must have 8 hearts with Emily and Shane, have seen the 6 hearts event and Shane's 7 hearts event. Enter the hospital on Tuesday or Thursday between 11 and 12 o'clock. This event won't show up if you are married to Emily or Shane.

Compatibility:

Please Remember My Birthday: compatible.
Stardew Valley Expanded: compatible.
Town Gossip: not compatible unless you delete the Marnie-Lewis folder.
The Ranch Expansion: not yet tested.
Polyamory: compatible.
Boarding House: compatible.
Town School: compatible.
Ridgeside Village: compatible.

Note that if some of your mods edit maps, the NPCs might break through objects or stand in places where new objects were added.

Special thanks:
Thanks to petriatnexus for helping me check the dialogues, to Lucypandyleo for helping me create Athanaeri Corner and being my pixel art helper, to Lemurkat for answering my doubts about modding and to richardtrle for helping me writing some dialogues! :)

If you enjoy playing with my mods, please consider buying me a coffee

This is the first mod that I've ever made and English is not my first language, so if there's any bug or some dialogues are not well written, please let me know. I hope you enjoy even more your experience in Stardew Valley with this mod!

Life Cycle - Rival Heart Events