So, the real likelihood is that Trump is just a really rich guy who loves being the center of attention, and is willing to pay for it. But here's another possibility that I haven't heard anyone suggest:
Trump is actually the Republicans' answer to Bernie Sanders. The Republicans can see the utterly unexpected degree to which Sanders' populist message is resonating with people, and they are deathly afraid that their own supposed populists, the teabaggers, will hear him and respond to what he has to say. So, their only possible response (short of actually doing something that benefits anyone but the Koch brothers, but let's stick to reality here) is to find a candidate that can keep those people on the reservation. Well, Trump is the perfect choice- he loves the media coverage, and he knows that he won't get the nomination, so it's just a fun carnival ride for him. And they can keep him in the news as long as Bernie Sanders continues his surge. Because, to a Republican, the notion of a Presidential candidate actually getting people to support him by putting forth an honest, middle class oriented message, is totally beyond their comprehension.
Then, when Bernie withers like all of their own one-month-wonders do, Trump can get bored and go home. That's the plan, anyway. Because, as I said, the one thing they never thought of in their worst nightmares is an honest politician.