Can think of several options (most of really like when a girl walks in my clothes)
From the standpoint of psychology. Girl using my clothes as if it "belonged" to me as my things are associated with my personality, that is, it as it is part of the "me." Also, putting on my clothes, girl enjoys tacit approval on my part, that shows trust. Plus the fact that she wears my clothes, speaks her location to me, well wouldn't a self-respecting woman wear something, which stinks, or the person does not like, which also gives confidence.
From the point of view of aesthetics. Usually men are larger than women in the shoulders, and even if the partners are similar in body type, men's t-shirt looks more on the girl. To see something fragile and beautiful, covered with large and rough tenderness, and looks quite romantic.
From the point of view of eroticism. To see naked women's legs peeking out from under baggy clothes - well, almost any man will carry the roof. This last time the girls popular shirt covering the buttocks - they are on the beach, and summer in the city you can walk, catching admiring glances...
What is Your opinion about this??