He has been spoilt rotten perhaps, growing up, or....he was deprived of love and attention and is making up for it now, by denying himself nothing.He is used to saying ‘jump’ and people jump.
I think he seems very thin skinned and wants his own way on everything, and is petulant like a child, and bears grudges.I don’t think he is insane, but quite savvy and knows how to be top dog.
All the above do not preclude him from being a good President ( Jack Kennedy was similar in many ways) but had more charm and intelligence.However, with Trump, you just know that he won’t be a good President!