#1 It was time for him to step away and let the new blood shine
One thing fans had noticed throughout The Rock's career was that he was an incredibly selfless human being. He had zero problems with putting anyone over. He lost three straight WrestleMania bouts to Austin, Triple H, and again Austin in 1999-2001. Fans might also remember The Rock putting over The Hurricane during the buildup to his WrestleMania match against Stone Cold in 2003.
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The Rock's second run in 2011 saw him take the spotlight on several occasions, and rightfully so. He was a big name at the time and it was a smart financial decision on WWE's part to let him take the centre stage. But maybe, his selfless instincts kicked in and he realized that it was time for him to let the new blood take his spot.