He quit because he faught with goldberg in a fight that he was supposed to win. But goldberg never followed the script and ended up destroying brock lesnar. so brock lesnar quit because he got mad. he then went to the UFC did well for a while when he started doing bad he quit and went back to the wwe. then he quit the wwe again.
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The easy answer is money.
After having a successful run the UFC, Lesnar found his career cut short by constant bouts of diverticulitis. The WWE offered a limited date, one year contract for a hefty sum, which Brock happily accepted.
In 2004 Brock Lesnar was fired from the WWE. He did try out for the Minnesota Vikings but failed to gain a roster spot. He is now in the Ultimate Fighting Championship.
ANSWER
after he left the wwe he wrestled at a few small organizations then tried the nfl wich didnt work out and he is now the heavyweight division in ufc
He had problems with WWE Management and that evolved into Vince McMahon pursuing legal actions against him, so he felt best to just leave the company for other opportunities.
I'm not sure what's wrong with Brock Lesnar, but if I had to take a guess I'd say roid rage.