Ego, first and foremost, but also perhaps a few other reasons is my guess:
He already has almost every other prize a man can have, money, fame.....
Besides, what does he have to lose? The people who hate him, or merely think him a buffoon won’t think any differently if he makes an execrable showing and is laughed out of the race, the people who love/like him will not think any less of him if he ends up being much worse at debate and/or running an actual campaign than he is at stirring things up.....
I think, also, he DOES have a few actual core beliefs and he is, on the right, like Saunders on the left going to consider it a ‘win’ if he forces everyone else to address these.
He can spend 2 billion dollars running and STILL have more money than I could ever imagine having, so why not?
Trump doesn't like to lose. If he thought he would lose, he wouldn't be in the game. If it were ego, and he just plain and simple wasn't concerned about the $$$, he would have run long ago. Let's face it: Other than Cruz, the field this year is extremely weak - the same old ho hum GOPe CoC candidates running - nothing of them at all exciting. And America is tired of the same old same old from Repubicrats. He gains NOTHING to run just for his ego, and stands to lose that AND $$$ if he loses.