Since I've been hanging out here and associating with more trans people than I ever have in my life it seems to me that people transition because of one or more of the following:
1) Their body dysphoria is too great to continue without some radical changes
2) They want to be seen by society as their true gender and they're willing to take the risks to do it
3) They felt they were "born in the wrong body" and will do anything to be their true gender
There may be some more but those are the predominate ones I see.
The problem for me, personally, is that I'll never have the "perfect" male body and I'm otherwise pretty happy with my life. I'll always be male inside, but the way my life has taken its twists and turns and ups and downs and given me the mindset, for lack of a better word, to deal with my situation without wanting to transition. I see that transition is a perfect option (or perhaps the only option) for some people, but I've never wanted that. I have my cozy little life carved out and am basically able to be myself regardless of what gender general society thinks I am. I sure as hell don't want to put myself through pain, medications with side effects I don't want (not to mention that I don't actually need them to live), family drama, life upheaval, etc. Screw that. I can also count on one hand the times in my life that I've had to truly "act female" (in other words, definitely look the part, wear a dress/make up, etc. etc.) so I've never been stuck in a relationship with someone who demanded I be something I'm not. I've never had a job that is biased towards female-bodied people. My parents basically raised me as male - allowing me to do whatever I chose to do as far as activities, etc. I had male clothes when I was a kid, whatever toys I wanted, and my mom rarely made me "dress up". So I think I had it pretty easy compared to some other trans people. When things got rough, it was because of my own dysphoria and my own bad decisions that made me spiral a bit out of control. Transition wouldn't have helped me then and it's definitely not going to do a damn thing for me now, so I've chosen to stay as-is.