According to the emblems that you can read on your way to High Hrothgar and the wiki, Paarthurnax started realizing that the way Alduin and the dov, for that matter, treated the mortals was wrong. He pitied men and with persuasion from the goddess Kyne, he started teaching mortals the Thu'um and the rest is history. It is obvious that he and Alduin were close, as any normal brothers would be, and he is saddened by The World Eater's fall.



Some speculate that Paarthurnax's intentions were far from noble and that he did all he did in order to gain power over the dragons for himself and would start terrorizing Tamriel just like Alduin, but I do not stand by this theory, because he himself says that he is going to teach dov The Way of The Voice to teach them to suppress their violent instincts and at some point in the game, after you have finished the main questline, you can run into dragons that aren't hostile to you and in fact help you in combat if they happen to be around while you're facing an enemy.

And yeah, I love the guy too. One of my favourite, if not my favourite, characters. <3