The New York Times has posited this theory, arguing that his humiliation at the 2011 White House Correspondents Dinner so infuriated him, that he decided to run just to stick it to the president.
The woman who sat right next to him that night has a different perspective. She wrote about it in this morning's Washington Post.
Reading this article reminded me of that whole Birther fiasco that Trump was leading in 2011. I watched part of his speech last night, and he was in full-voiced "nanny nanny boo boo" bully mode against Hillary. She should study the way Obama tore Donald apart with humor in 2011. For every "she's old" or "she screams" or "she plays the woman card", Hillary should say, "Why don't you run along and bring us that Obama birth certificate bombshell you promised."