Yes it is a choice, and that's why it was an obligation you chose to undertake, not a duty someone put upon you. That's the difference.
If you're a christian, then obviously it's not a virtue. I don't operate under the christian value system however and selfishness is a virtue under the one I do follow.
Look at it this way, when one fights for their country, should they do it for themselves or out of a sense of obligation to others?
Both.
You do not get to choose what is or is not a virtue. Virtues, morals, values are culturally ascribed and not decided upon by individuals. Perhaps you should return to school and educate yourself.