Identity: The individual characteristics by which a thing or person is recognized or known.
There is some innate desire by people to be able to identify with someone/something at all times. Ever noticed how the minute you meet someone new and have the slightest information about them, you are trying to associate something or the other with them? Common people from the same school/college/university/work place? Once this is established, it changes nothing unless the middle person is a close friend of the both of you in which case you would have heard about one another in any case. Otherwise, you just speak about this common person for a few short minutes fueling the conversation and that’s about it. What is it then that makes us seek out this connection? You just feel closer, more connected to this new acquaintance for however long you’re going to see them. It’s totally weird and yet totally true. And it's almost always one of the first things to be discussed in context. It probably gives one a sense of familiarity and increases the appeal of the person to be associated with someone we like. On the other hand, we are wary of those associated with people we don't like. It acts as a mental filter of sorts and lets us set our guard at the level we deem appropriate.
Bafflingly though, it's easier perhaps to form an association with someone you have absolutely no one or nothing in common with. This helps you to finally be able to judge them as they are for what they are worth with your own standards with no inputs from any past experiences. And it gives everyone a fresh slate to start with. And almost everyone deserves that. And if they didn't, it's bound to show up soon. Like Ron (Harry Potter character) sagely says, poisonous toodstools don't change their spots.