For me it’s interaction. Someone who likes a giggle and shows acknowledgment to my posts, if you are a watcher then I won’t follow. Simple really, why follow when the person doesn’t want to know you! ??Follow accounts and why?
Do you follow accounts you have a genuine interest in? To perhaps chat more intimately / privately?
Social interaction ie through alerts and notifications etc?
Simply they’ve caught your eye and you want to get to know them better perhaps? X