Not sure...usually commonality, similar interests and such.(there doesn't need to be alot but at least one with that connects us, be it a place or and interest for some to build) Its kinda if we click and our personalities don't clash and we accept each other for who we are then we're pretty much friends for life. Hence I've kept the same close friends for half my life now weirdly were all quite different...ride or die type bitches lol