Back when I was a premed, I wanted to one day be the one who made the "big save" as glorified by medical TV dramas. Who didn't? As much as altruism and selflessness plays a role, heroism and ego are deeply intertwined. It also takes a certain amount of hubris and self assurance (grounded or not) to believe that one could even play the part of the "heroic doctor" down the road.

The combination of med school and residency and time itself changes us and our motivations. I started residency being interested in critical care, and while I did enjoy my ICU rotations, I've since then found my interests to lie in the outpt world, where I can quietly go about making small incremental changes (undramatic, but the longitudinal care fleshes out my pts more). Also I recommend Atul Gawande's New Yorker article on incremental care. So while ego played a part in starting down this path, like many others in this thread, I think the responsibility (powerful and burdensome) of the job now confers with it a certain humble respect that I could never have appreciated in the beginning.