I have been a life-long crossdresser (seriously, I remember wanting to wear girls' clothes since I was 4) but I am not transgender. I actually enjoy life as a sterotypical guy.
I consider myself as a "collector of feelings", meaning that I want to experience more or less anything (positiva) that any human can experience. One of the reasons why I love women's clothes is the variety of fabrics, how light they are, etc... So, while vaginal feelings ae out of the way (even if I ever had vagynoplasty I am sure it wouldn't feel the same as a woman's), boobs seem to be a more reachable target for some new feelings. The weight, the bounce, the softness... I am curious. And they'll look great in a bra or a corset
The other thing is that my nipples have always been 10x more erogenous than my "downstairs", so a bit of enhancement in that area maybe coul dbring some extra pleasure (it does indeed).
Gynecomastia seems to be a common thing among men of a certain age. I have never been super thin, and last year a doctor actually diagnosed me with gynecomastia, so... why not?