#4 Previous defeats
It takes a lot of time for old wounds to heal. Brock, on a daily basis, is reminded of the beatings he took at the hands of Alistair Overeem and Cain Velasquez. To be more specific, the latter gave him a scar underneath his eye that is still visible in the present day which must get to Brock when you really think about it.
After all, he proved against Frank Mir that he loves gaining revenge.
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Learning from your mistakes is a big thing to do, and the losses will mean something to a man who prides himself on being the best at everything he does. No matter who you are, being humbled like that can take its toll and fighting back from that low point to return to the octagon is something Lesnar should be applauded for.
Speaking of which, courage plays heavily into our final point.