Trump has toyed with running for President for several years, and of course this year really jumped right into the ring. Early speculation was that he'd stay in until time for the required financial disclosures, then bail out. But he didn't. He filed and continued to march.
No one knows what he was thinking, but perhaps he was willing to "take a shot" at running, just for the hell of it, basically. If he ran for a while and flamed out, well, at least he'd be more famous and enhance the value of his brand.
(Digression; I don't think in terms of "brand." I know it's a real thing, and people do think that way, but not me. So to write "enhance the value of his brand" feels odd to me, like having pebbles in your mouth. Anyway...)
Trump may have considered it a win-win. Either he'd promote himself spectacularly, or he'd actually get the nomination. If he got the nomination, then his self promotion would go to a whole new level - the best, incredible - and if not, he'd still have made himself more famous. What's not to like?
But I suspect he failed to anticipate or appreciate the level of scrutiny he'd receive. As a wealthy bully, Trump has gotten away with some specious behavior, but no one called him on it. You've seen the type. Wealthy bully. And you know how people act around celebrities, even minor ones, and rich folk. Fawning. Making allowances for outrageous behavior. Enabling in order to bask in reflected glory.
So Trump isn't getting away with everything so much anymore. And the truth is coming out. And it's ugly. And his brand is suffering. His brand is now described by Huffington Post thusly:
Editor’s note: Donald Trump regularly incites political violence and is a serial liar, rampant xenophobe, racist, misogynist and birther who has repeatedly pledged to ban all Muslims — 1.6 billion members of an entire religion — from entering the U.S.Can't be good. So Trump's hubris caused him to fly too close to the sun, the bright hot light of fame and publicity, and it exposed him. Watch now as he plunges, like Icarus, into the sea.