Here a some ways on how your characters could become/could've become friends.
"School & Childhood" Friends
Sitting next to each other on the first day of class
Being assigned as lab/project partners
Having the same teacher and complaining together
Being the only two people who don’t know anyone
Riding the same school bus
Getting seated alphabetically near each other every year
Being in detention together
Letting the other cheat off of them
One person defending the other from bullying
Meeting at recess or on the playground
Competing on the same academic team
Being roommates in college
Always sitting next to each other in the library
Staying up late talking in the dorms
Helping each other study before exams
Meeting during orientation week
"Family Connections" Friends
Their mothers/fathers being friends
Being cousins close in age
Family friends who vacation together
Parents arranging playdates
Siblings introducing them
One family helping another during hard times
Their families attending the same religious or community events
Becoming friends because their kids are friends
Meeting at weddings or family reunions
Dating each others' siblings
"Neighborhood & Everyday Life" Friends
Living in the same apartment building
Seeing each other walking dogs every day
Helping shovel snow or carry groceries
Meeting at a local park regularly
Frequenting the same café
Running into each other during morning commutes
One person helping after a car breakdown
Being regulars at the same store
Sharing a laundry room in their apartment building
"Work & Professional Life" Friends
Starting work on the same day
Complaining about the boss together
Working late on the same project
Covering shifts for each other
Sitting beside each other in the office
Traveling together for work
Bonding during training sessions
Accidentally getting trapped in endless meetings together
Quitting the same toxic workplace
"Shared Interests" Friends
Joining the same sports team
Meeting in an online game
Taking the same art/music/dance classes
Meeting through cosplay or conventions
Participating in hobby communities
Posting in the same niche online forum
Volunteering for the same cause
Following each other online
"Through Other Friends" Friends
“You two would get along”
Becoming the last two awake at a party
Mutual friends inviting both people repeatedly
Being added to the same group chat
Becoming friends after a breakup divides the friend group
One friend “adopting” the quieter person
Meeting through double dates
Their two friend groups mixing with each other when two people start dating
Becoming roommates through mutuals
"Random Coincidences" Friends
Being stranded somewhere together
Wearing the same outfit or shirt
Reaching for the same book in a library
Accidentally texting the wrong number
Dancing next to each other at a concert
Sitting together in the airport during a delayed flight
Sitting next to each other on the train/plane
Getting stuck in an elevator with each other
Meeting on holiday together
One person dropping something and the other helping
Surviving an embarrassing moment together
Laughing at the same thing no one else noticed
"Funny & Unexpected Situation" Friends
Mistaken identity leading to conversation
Fighting first and later becoming close
One stealing the other’s seat
A shared hatred of the same person
Both pretending to understand something they don’t
Meeting because of a lost pet
Becoming friends after a minor car accident
One person overhearing the other say something unhinged and agreeing
Meeting through a heated conversation in a comment section
Accidentally attending the wrong event together
Competing against each other and gaining each other's respect
More: Friendship Masterpost (Prompts & Advice)
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