Just because you chose a job that might be construed as "heroic" doesn't mean that's why you chose that job. And bear in mind that when we talk of these upper echelon jobs like astronauts, there might be people queuing up to do it, but the people queuing up aren't random civilians fresh out of university looking to start a career. Many of them are already doing courageous and selfless things, and are looking for further that desire to help and better mankind. They already are heroic, to some extent.
Your "sought" vs "thrust upon" contrast doesn't seem to really be a very good comparison. One suggests a lack of choice about the situation, but the other doesn't just indicate there was a choice, but rather that the person desired to be heroic.
My understanding of heroism is that it occurs when a person is presented with a choice of either turning from danger, or confronting it, and confronting danger primarily benefits others, rather than them personally.
So for example, the passengers on UA93 are heroic because I suspect they had a pretty good idea of how slim their chances of successfully seizing the aircraft were. The probability was that, either way, they were dead. However by directly engaging danger, instead of "hiding" from it by passively remaining in their seats and hoping for the best, they forced the hijackers to crash the plane and therefore saved the lives of those who would have been targeted by the flight.
In contrast, the crew of US Airways Flight 1549 were not heroes because in trying to safely ditch the aircraft they benefited just as much as the other passengers.