I've heard that Augustus originally wanted an "Augusteis" from Vergil. I know that in the end Augustus really liked the Aeneis, but is there any known reason why exactly Vergil decided to write an "Aeneis"?
My impression is this: Augustus was a real and contemporary person, so it would be difficult to credibly fabricate a story of his works, adventures, and greatness. This restraint was removed by writing of his remote ancestors (Anchises, Aeneas, and Ascanius/Iulus from whom the Iulius family allegedly descends). Then there is a freedom to fabricate and bend some facts due to the historical distance but Augustus could share some of the glory by being a "direct heir" of these great and mythical men.
However, I have no proof of this. I may have read something along these lines long ago, but I am not aware of any existing evidence, so there may be nothing we can do besides speculating. (I would be happy to be proven wrong, though!)