#1 Why he will win the WWE Championship: He has already lost multiple times this year

Brock Lesnar lost his Universal Championship at WrestleMania 35. Then he won it again before losing it back to Seth Rollins. These two losses have hurt Lesnar a lot more than the past few years combined because Rollins also beat Lesnar clean at SummerSlam 2019.
Whenever Lesnar returns to WWE, he creates havoc in the ring. This time, he attacked Kofi Kingston and it doesn’t seem like he’ll lose to the latter.
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The betting odds are also in The Beast's favor and since it was Vince McMahon's call to bring Lesnar back to SmackDown, he may not let him lose this match. No offense to Kingston, but if he beat Lesnar (presumably clean since he is a babyface), it would damage the former Universal Champion's credibility forever.
And since Lesnar has lost a few times this year already, losing another match doesn't make much sense. Talking about Survivor Series' Champion vs Champion match, it wouldn't surprise us if The Fiend loses his match via DQ against Rollins as he did during a recent live event. This way, he could confirm himself a ticket to WrestleMania (or Royal Rumble) where he will win the title.