Board Thread:General Discussion/@comment-11276487-20130721020114/@comment-11344160-20130722234732

Alduin is basically just a metaphor for Jormungandr, the creature which represents the end of the world. It's a myth, nothing anybody bothered researching or figuring out because why bother? Myths don't come true. So who knows what drove Alduin.

Based on what Paarthurnax was saying my guess is that Alduin wanted to usher in a new world where the dragons once again ruled, free of stubborn 'man'.