Quite some time ago, Krauthammer, the former psychiatrist, called Trump a narcissist and he wasn't using the word loosely.  Anybody can Google "DSM III Narcissist" and read the diagnostic criteria. 

There is a difference between a social metaphysican and a narcissist.  The narcissist has an internal view of themselves that is grandiose - and out of that they build a wall of pseudo-self-esteem and live behind that wall - it gives them their strength, their defense, and their self-image.  As you can see, Trump, from behind that wall has been able to suceed quite well in the business world, and with relationships - including family - (which is unusual for a great many people with this kind of defense structure). 

The best metaphorical description of a narcissist is of a person who lives inside of a gold-plated egg-shell.  No matter which way they look, they see this golden reflection of themselves.  The only real dissonance is from criticism from outside, and that is why it will be extremely hard, and feel unnatural, for Trump to not fall for the traps the Clinton campaign sets - they attack and he will feel compelled to counter-attack.  His counter-attacks feel absolutely necessary to keep the golden image in place.  An unanswered attack is like risking EVERYTHING - his universe, his very self. 

What he might learn (or have already learned over the years) is to pretend to play a part where by not counter-attacking, he is sneaking up on them over time and will get them later (that's anyone's guess as to how well he could do this) - but whatever he does, not counter-attacking will take enormous will power and will seem very strange to him. 

The standards by which he decides he is under attack are different from what we might imagine because his standard of value is so different... it is his golden reflection.  By this standard, a very minor slight from someone that he assumed was on his side could set him off, but a viscious attack by an enemy might not.  Seeing everything from this perspective means he has far less ability to empathize - at least that is so for most narcissists, but for Trump, he does have a record of considerable loyalty.  He is very unusual.