HE said it himself "I'm in this to win"
His former campaign strategist said he never intended to actually become president, he was only using this election to point out the issues. But after he got so much support, he became a runaway train due to his ego.
i don't think he really wanted to be president now he's backed into a corner with responsibilities he's doesn't really want to have
I have always suspected this was all about Trump's ego. He wanted to win, but he never actually wanted to BE the President. That's my guess. I don't think he actually thought he'd win, and now he's probably thinking, "Crap, now I have to be President." I wonder if he has any regrets. I'm sure his ego is through the roof right now, and maybe it's enough to pump him up and make him think he could actually do it. But I wouldn't be surprised if he resigned at some point.