its been a gradual process for me, never a big epiphany.
I was drawn to medicine for a number of reasons. I have always liked science and thought it would be cool to be able to apply science in a way that was also productive and was helping people out. I do like helping people out. Surgery sortof chooses you-- if you are a person who likes action much more than words, likes using your brain to figure out why someone is sick-- and then going in and fixing it-- if you are task oriented person who likes using your hands.
CT I was drawn to because I loved the anatomy but I think I was also drawn to it because it was sortof like the Top Gun thing. it is the ultimate challenge in surgery and it draws people who are compelled to challenge themselves. the surgery is elegant-- and is much nicer than sloshing around in the belly with fat, poop and guts (sorry if that is too graphic). The heart and the lungs are really pretty amazing things to work on. I also like constructive surgery (sewing anastomoses) rather than destructive surgery (big cancer resections)...
with all that being said I'd rather be a stay at home dad if I could. but that isn't really in the cards right now... especially since "we" are in crap loads of debt accrued over the 9 years of training...