He's probably torn between wanting a successful run for office, and wishing the whole thing would go away.
And none of this considers the very real possibility that he's playing for Hillary to win, or that he was never serious to begin with.
I think he figured he'd grow his brand, have some fun, plan a nice way out (which may still be in place after he makes chaos of the GOP nomination), then a quiet retreat of "I feel I can do MORE for america by skipping the presidency and building this wall.... " which never materializes. He gets Rubio to promise him the wall contract, he quietly steps back and is the winner that didn't have to take the job. AND/OR, it's all about his wedding buddy Hilary winning the job.
Trump is NOT a hard worker. He will be MISERABLE in the 14 hours days of paperwork at the white house. He's a celeb with a solid gold living room who is 70 and doesn't work all that much these days. The white house is NOT as nice as his current home. The prez limo is NOT as nice as his own rolls royce, he's already said so.
Trump loves 3 things - making deals, building things, and public battles. Guess what, America... a president doesn't really do ANY of these 3 things. Congress makes deals. Governors build things. And TV pundits have public battles. Presidents are dignified spokesmen and cheerleaders who quietly make decisions in boring rooms surrounded with boring bald people, 7 days a week. That ain't trump.