Ever wonder why we have crush?

First thing in live,everytime we saw someone we might fell in love with them but do you ever wonder why? Cause i am.Everytime when a person past by i mean like anyone no matter how old they are or how different they are or where they come from,you might fell in love from the first time you saw them.I mean like anywhere. For me it call "first love at the first sight".For me why it call that when we in school we call it "crush". For me there is no word or disruption of a crush. But crush is just a definition of a *brief but intense infatuation for someone, especially someone unattainable* so that what it is.