It's not a question of being happy v. unhappy. It's a question of maximizing your market value.
Furthermore, people have a rosy picture of an athlete's life based on the money, and don't focus on the work they put in. I may not make as much as Tommy does, but he works a hell of a lot harder than I do. He works 14, 15 hour days, takes a physical pounding, and essentially crams all week for a mental exam at the end of the week. Go into an NFL lockerroom after a game. If you want to see some guys that take a beating, and keep on going, take a look around. They make a lot of money, sure. But they also have a very short shelf life, and their career is in danger every play of every game, every play in practice, and in the weight room. Is football fun? Yes. But there is a lot of hard work, and abuse taken in order to play the game. They would be silly not to maximize the amount of money that they make. Perhaps it's because I've seen it from both sides, but I laugh sometimes at how bitter and hostile fans get over player negotiations. It really is just business.