Reasons for not moving up...

Up until recently I was under the impression that everyone's goal was to move up limits as they acquired the skills and bankroll. But a few posts opened my eyes and I realized that there are a fair amount of people happy with where they are. I understand the pro's that make their money and don't want to jeopardize their earnings by losing at a higher limit. My question is geared to the people that do this on the side. What is it that makes good players decide they don't want to move up? I always thought of poker as gambling, and I think gamblers take some risks. I just want to understand the reasoning.