Brock Lesnars first run with the WWE was a case of too much, too soon. Upon debuting in 2002, he was immediately pushed to the moon and given the Undisputed WWE Championship. By 2004, only two years after his debut, he had already done everything there is to do, and he quit WWE to try his hand at playing in the NFL. It took eight years, a failed NFL career, and an infinitely more successful MMA career, but Lesnar eventually came back to the WWE the night after Wrestlemania 28. He recently re-signed with the company, so it looks like Lesnars next retirement will be much more permanent than his last one.
