Michael Phelps has nothing to prove to anyone, but neither did Michael Jordan, Kelly Slater, or Gordie Howe. Still, these legends all got up off their laurels, un-retired, and tried to compete at a high level. Some succeeded. Others, not so much.
Why he hung it up: The first time Jordan retired he wanted to play baseball, had won everything he wanted to, and was likely gambling too much to dedicate himself to the game.
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Why he returned: Because he's quite possibly the most competitive person on the planet.
His success: Jordan led the Bulls to three-straight championships between 1995 and 1998. He had decent personal stats with the Washington Wizards between 2001 and 2003, although the team didn't perform and he singlehandedly ruined No. 1 pick Kwame Brown.