There seems to almost as may answers to this question as they are members on this list. I share the most common reasons of feeling better, more relaxed, more comfortable and pleased with the way I look.
However, I'll answer this from a slightly odd or negarive aspect. A few months ago I had an eye infection which cause me to miss several CD dressing occasions, meaning a weekend and then a national holiday a few days later. I wear contact lenses when I go out as a CD but could not wear them for a few days and don't have female glasses. I spent these days doing what I would normally do dressed up as a CD, but now inmale attire. I felt utterly horrible, literally devistated. It was wrong, somehow. I envied the people around me who were going about there business dressed mosstly innthe informal clothes people wear on the weekend. I felt I was in some kind of straight jacket. The infection cleared up after about a week and when I went out again as a CD the feeling was fantastic. I was me again.
This is why I am a CD and why I dress.
"in the end you crossdress because you want to." This seems to be the bottom line. Why we want to has hundreds of answers. In the end result, it feels right and natural. Not dressing is somehow wrong and unnatural.