#2 His body has probably taken enough punishment
How many times have we heard a legend talk about how much they regret hanging on to the business and not leaving when they should have? Pro-wrestling is a demanding job, and it sometimes takes years out of a Superstar's life. A wrestler could suffer an injury during his career that would not only take him out of the business, but it would also affect the rest of his life after retiring.
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The Rock has wrestled hundreds of matches by this time, and his body has taken enough punishment. Superstars like Austin had to retire early because of injuries, and The Rock probably decided to leave before something like that happened to him. Rock's movie obligations also prevented him from putting his body at risk, and this could be a major reason why he left quietly.