Why does a crossdresser get aroused when dressed as a woman?

A male crossdresser gets aroused when dressed as a woman because he has fetishized female clothing. It is just like other fetishes. A more common example is how some people become very aroused by wearing a "school girl uniform." There's nothing inherently erotic about school girl uniforms, or any clothing for that matter, but people learn (usually unconsciously) to fetishize it - that is, to associate it with feelings of arousal. The same goes for females who crossdress as men.