The clothes being the obvious first point. The styles, looks, colors (especially the pinks) and materials are just so much more fun than male clothes. I love the process of dressing up and the feel of the clothes on my smooth body.
Also being submissive, controlled, and helpless. Being tied up and, for example, made to listen/watch hypnos is a big turn on for me (even when it’s self-afflicted). I enjoy locking myself in chastity too and having that cage around my clitty is a great constant reminder of my place and where I belong.
The thing that also excites me is the mystique of it. The idea that this pretty regular and masculine (I don’t think my frame would be passable) guy has this other, hidden, secret side is very exciting. Living my day to day as a normal dude but behind closed doors and curtains I can be someone and something completely different. My sissy me is in the closet and most likely will stay there and hiding my panties, chastity, stockings/pantyhose/thigh highs, maybe even a bra under my male clothes while I’m at work or outside is a very strong and titillating feeling.