Why are Metal listeners so critical about other genres of Music?

This is from an 18 year old girl.I LOVE metal.Heavy,Thrash,Death,Black...Most of its subgenres.Its my ultimate of music.But when I'm not in serious music mood,I enjoy listening to other genres.I might just blast Lady Gaga and Cascada in my car.I might just put on Adele when I want to slow down.And most of my 'metalhead' friends will start critisizin' and hatin' on those Artists and go like 'Oh you've put your sh!t music on'..Why's that? What's so wrong with such music? I seriously wouldn't like to live in a world where the only form of music is Metal!