Before I started my transition, I answered this question by exploring what it was that motivated me to dress up as female:
Was it the clothes? (not really)
Was it the sexual gratification? (didn't get any from it)
Was it looking pretty? (that was nice, but not important)
*Was it wanting to change my body? (yes, a big part of it)
Was it having a female identity? (woah, huge lightbulb moment here!)

Anyway, try asking yourself some of these questions, and see which ones light up for you, and you will have your answer, or at least be closer to one.

One parting thought: I've always thought of transition as being not about what gender you are, but who you are as a whole person. Everybody goes through various transitions in life trying to find out who they are; mine just happened to require that I change my gender in order to do that. I transitioned to be me, not to be a girl. It just so happened that that was the direction my transition took me. My identity is continually in transition; I've transitioned from a student to a professional web developer; I've transitioned from a mentally healthy person (or so I thought) to a person with Bipolar Disorder; I've transitioned from a passive person to an assertive one. I've transitioned from someone who made decent money, to someone who is extremely poor, and now I'm making the same transition in reverse.


*does not necessarily include all major body differences, including genitals. Many trans women opt not to grow their breasts too large, or choose to keep their male genitals.