According the PBS Frontline doc, Trump's reasons for wanting to run for President (this time around) can be traced back to the White House correspondents dinner in 2011 where he was thoroughly roasted and humiliated by Obama. Since Trump is the kind of person who never gets talked down to or put in his place, he vowed revenge on everybody who laughed at him by proving them wrong and showing he could actually run for President.
If this leads to the collapse of the current GOP and their pandering to the lowest dregs of society, Obama should be raised to sainthood.