It's rare that you go out on top in professional sports. It's even more rare to do so in combat sports, especially mixed martial arts. It's not just the fact that GSP went out on a win, or as the UFC welterweight champion. He also has not a single unavenged loss on his record. Both men who defeated him, Matt Serra and Matt Hughes, would go on to lose to St. Pierre - which means that, though they were able to beat him once, he was able to prove that he was equally able to beat them. In fact he would beat Hughes twice, and Serra was never the same fighter after facing GSP the second time. Why come back and risk that? While his career wasn't perfect, MMA is arguably the hardest sport to go undefeated in, and GSP fought the best of the best throughout his career. It's as perfect as it's going to get, frankly.