@Icon,
Are you investigating the meaning of virtue or are you asking what motivates people to be virtuous?
If the former...
I think it's hard to beat the ancient concept that people are virtuous to the extent that they practice temperance, prudence, justice, and fortitude. Then you may want to add the Christian concepts of faith, hope, and love. Of course, this just puts you in the position of having to explore the meaning of these seven words within the intended context--which tends to be a very interesting exploration.
If the latter...
I think the answer depends on your assumptions concerning who we are, and where we are going.