Glad to see the new forum--even though I originally voted "no" when I first read the poll (before plowing through that whole thread initiated by iaboy). This forum gives this point of view a legitimacy that has repeatedly come under fire for a long time (at least as far back in 'time' as I have gone through the archives of the first iterations of BN.)
It seems to me a big part of the conundrum here (the topic of this thread) is that many construe "transgender" to mean there are only two choices in the gender department: male or female. From this perspective, if you were born with testicles and say you are transgender, that means you want to hack it all off, grow boobs and get a neovagina. I think you can figure out the converse for GG's.
So, if this is what anyone asking (me) means by transgender, then no I am not transgender. However, as with literally EVERY other aspect of the human body or human condition that can be measured or described, gender isn't a matter of two mutually exclusive states. Gender is in fact, multidimensional and multifactorial (google it before you ask, please.) Gender in the simplest sense, is a spectrum with completely 'male' and completely 'female' forming the endpoints. However, while that is two dimensional it does simplify the discussion.
Thinking of gender identity as a spectrum, I definitely do not fall squarely at either endpoint, and in my mind, I am transgendered. I have never been unhappy with my male bits, yet I have envied women their bits from a very young age. Having breasts now, while still outwardly presenting as 'only' male has eased my GID tremendously. I think I had/have 'GID' mostly because I was never happy being 'exclusively' male, yet I didn't feel unequivocally female either. My dysphoria arises from feeling pressured into having only those two choices. Come to think of it, it's not even a choice--this is just how I have always been for as long as I can remember. Considering everything I have had to endure feeling like this, I would NEVER willingly 'choose' this path.
Transitioning never seemed like the correct option even before I started NBE. Despite having almost a full b-cup at their biggest (I am on a break and they have reduced somewhat in size) I am still convinced transitioning is wrong for me.
The problem as I see it and a large part of the reason I was/am dysphoric, is that societal convention seems to require that I must either choose 'male' or 'female'. Well too bad, because I'm not either one exclusively.
The folks who complain the loudest about men like me (the feminazis and the transnazis) are completely stuck in the idea that gender is a binary condition with mutually exclusive states; to wit, the MTF transitioners who proselytize that I and anyone like me are in denial of my true identity and advocate no 'man' would ever want breasts; and the GG's who volunteer to speak for ALL women the world over stating that no woman would ever possibly find a man with breasts attractive or desirable. Well, I'll have to run right out and tell my live-in girlfriend of 2 + years because apparently she didn't get that memo.
So, back to the OP's question: why only breasts? Actually it's not only breasts, but growing breasts is pretty much the only thing I can change at this point in my life without going under the knife, which I am not willing to do (either to transition or reduce my 'gynecomastia'.) Are there other aspects of my life and self in which I express my 'trans-ness'? You bet, but this forum's focus is NBE. i would be willing to bet that every man that has managed to grow feminine breasts has made similar changes in other parts of their lives, so in reality it isn't just breasts. But it also isn't about transition. For me, it's actually been more a path of self-actualization: the desire for self-fulfillment, namely the need to become actualized in what I am potentially. (from Maslow) Rogers stated it as "man's tendency to actualize himself, to become his potentialities... to express and activate all the capacities of the organism."
Anyway, sit with that and see what comes up for you. Feel free to disagree, but I have already rejected your reality and substituted my own.