A either always sits in the exact same seat every day, OR he's on a [train/plane/bus/etc] that has assigned seating based on your ticket. he gets to his seat and finds B sitting there. A does his best to talk B into fucking moving already, but B refuses to see reason. B finds how worked up A is over this to be hilarious/amazing/kinda-sexy-wtf.
semi-au! A is still himself, aka the idol/actor/model of your choice. he either gets injured or needs to limber up for a [variety show/tv or film role/dance moves bc he's not that good at dancing] and so the company brings in B, a [physical trainer/yoga instructor]. A's first thought upon seeing him is "oh no, he's hot".
A's minding his own business, walking his cat, because the universe hates him. he bought a cat because he specifically didn't want to have to walk his pet, he somehow managed to find the only walk-loving cat in existence. A works crazy hours, so the only time he has to walk his cat is like 4AM. On one of his walks he runs into B, who's walking his dog, a totally normal thing to do. B visibly and verbally mocks A for walking his cat like a weirdo. Bonus points if B is a baker/chef of some kind, and thats why he's up super early.