Trump simply cannot help himself. Like a child. Or an addict. Unfortunately for him, he is incredibly thin-skinned. He is super-sensitive. He cannot hear even the slightest challenge or criticism without coming out swinging like a ranting, raving, unhinged, emotional lunatic, which sends him careening off message.
For some strange reason, many American citizens today seem to believe that because an individual may have come from a privileged background or a 'political' family, they should either have a right to attain elected office or will naturally do a better job than someone who isn't 'privileged' or part of a political dynasty.