So what if it's a habit?

In class this week, I was slightly bewildered by one of Aristotle's beliefs/assumptions: that if a person is performing a virtuous act by habit that it does not make him a virtuous person. An example in class was a soldier in the military. A soldier can be trained to save people's lives. So every time he saves a life because it is his duty to do so, it does not count as being virtuous? What about the fact that he's even in the military? Maybe he joined because he is a virtuous person, and in the eyes of the solider, joining the military was a virtuous act in itself and later on, after training, it became a duty, a habit, and maybe even a natural reaction to save lives.

I disagree, and hope to understand more of how Aristotle's thought originated, or gain a better perspective from his point of view.