My theory on why it isn't cool to wear the T-shirt of the band you're watching is simple. We know you like the band, asshole, you're at the fucking concert! I like meeting people and talking about music at concerts, so if people where t-shirts of OTHER bands they like, it's easier to make conversation.
Strangely, I don't hold this same rule when it comes to sporting events. Yes, I know everyone at Wrigley Field is a Cubs fan, but for some reason, it doesn't bother me that they are all wearing Cub clothes.
Call me a hypocrite
