I remember that as a kid I LOVED getting hand-me-downs from my best friend (who was actually the same age as me). My affluent parents were bewildered, because they had both suffered hand-me-downs as children and hated it, and as the eldest child I need never have had hand-me-downs at all, yet here I was actively going looking for them.

The basic idea behind it is that when I put on my friend’s clothes, it was like some of her coolness was rubbing off on me. Plus, wearing your friend’s clothes just makes you feel closer to them, because if you’re with your best friend and then another kid comes by, you get to boast “I’m wearing his/her shirt! We’re best friends!” and it kinda excludes the other kid from your little circle of friendship. If I was a psych major I’d go the full nine yards and say the behavior is due to the subconscious desire to get inside their “skin” but I think I’ll just leave it at: wanting to emulate the person you like in a very basic way; by wearing their clothes.