Why do I like it when girls dress up... Especially my GF?

I don't think there is anything weird about that at all, and no reason for you to feel bad. The main reason girls dress up is to look more attractive, and I think that you are not alone in thinking that nicely dressed women are a pleasure to look at! I feel the same way sometimes, and I have a few friends who are just like you and really love it , though personally I tend to be more partial towards girls who just finished working and are sweaty and covered in dirt lol. Everyone has their tastes!