If you need a pair of jeans, it is customary and often expected that you go to a number of different stores and try on a bunch of pairs. You feel them out, see how they fit, if they look good on you. You might try different styles, different colors, different textures. Some look really nice and clean, others are more worn and rough around the edges.
There are so many options and you keep looking until you find the right one.
Sometimes, often when you least expect it, you see a pair of jeans and you know you need them. Those are your jeans. You don't even really need to try them on, you know before you try them on that they were meant for you.
It is often the same with friends. Over time, you can become acquainted and build a strong friendship with just about anyone. But some people, for no apparent reason, you instantly have a connection. It is like you already know each other, you don't need a lot of explaining or justification.
It is kind of like that.