He needs his art to be successful, because he feels a responsibility for his employees.
One could also argue that he needs his art to be successful because he's human.
When I post a pretty good photo to Reddit and it gets downvoted into oblivion for no good reason other than The Internet, that kind of hurts. So I imagine that when Lucas gets completely and brutally savaged for Episode 1/Jar Jar/"I Hate Sand"/"NOOOOOOO"/whatever, that really has to sting. Being that he's had to deal with fans all his life, he's probably gotten better at compartmentalizing that savage beating (plus 4 billion dollars soothes many wounds), but as an artist, it's gotta still be a blow to the ego/pride/emotions.
***EDIT***
That's also why I cut George some slack on his comments of how he wished they had asked for his input and how that wasnt the movie he would have made. Sure he has had other projects in his career, but really, for the last 45 years, Star Wars has been his life, his baby. Its one thing to walk away, logically thinking "Yeah, I'm done with that", and another to actually do that.
But as an artist I know there will always be this little spark, an emotional attachment to a project - any project. Four billion dollars or not, I can easily see him second guessing himself and regretting putting that part of his life on a shelf. It can be tough to walk away from something and see it in someone else's hands.
Edited by Desslok