Why is Donald Trump running for president? Marc Hodak nails it. Trump is planning to auction the Presidency to the highest bidder.
Sure, he may be happy to win the presidency. He can go for it, and see how far he gets. But what happens when it becomes plain that he can't win, that he is only polling ten percent of the vote, or five percent of the vote? No matter; he still has what every dealmaker wants--huge negotiating power.
If his support would largely draw from a Republican candidate, he can go to, say, Jeb (or whoever wins the R slot), and propose to drop out if Jeb gives him X*. Or, he can got to his old pal Hillary (assuming she wins the D slot), and propose to stay in the race if he gets X.
The art of the deal is to have something that other people desperately want, and get them to bid on it. Imagine holding the keys to the White House, and the top candidates have to get them from you.