Trump

Trump's personality disorder

One of the main reasons why Trump is unfit to become president is that he has all symptoms of a personality disorder, Narcissistic Personality Disorder (NPD). As a matter of fact, Trump is a textbook case of NPD, so blatant that you don't even have to interview the patient to come to the diagnosis.

Narcissistic Personality Disorder

Many psychiatrists will use the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) , published by the American Psychiatric Association, to diagnose mental conditions. DSM-5 describes NPD as follows:

A pervasive pattern of grandiosity (in fantasy or behavior), need for admiration, and lack of empathy, beginning by early adulthood and present in a variety of contexts, as indicated by five (or more) of the following (p.669 ff.):

Has a grandiose sense of self-importance (e.g., exaggerates achievements and talents, expects to be recognized as superior without commensurate achievements).

Is preoccupied with fantasies of unlimited success, power, brilliance, beauty, or ideal love.

Believes that he or she is “special” and unique and can only be understood by, or should associate with, other special or high-status people (or institutions).

Requires excessive admiration.

Has a sense of entitlement (i.e., unreasonable expectations of especially favorable treatment or automatic compliance with his or her expectations).

Is interpersonally exploitative (i.e., takes advantage of others to achieve his or her own ends).

Lacks empathy: is unwilling to recognize or identify with the feelings and needs of others.

Is often envious of others or believes that others are envious of him or her.

Shows arrogant, haughty behaviors or attitudes.

DSM-5 says 5 of these suffice to diagnose the patient as NPD. Trump has them all! Let's have a look.

Asked on MSNBC’s “Morning Joe” who he talks with consistently about foreign policy, Trump responded, “I’m speaking with myself, number one, because I have a very good brain and I’ve said a lot of things." Enough said.
Trump wants to become the most powerful person on the planet: the president of the United States. Well, others might have this dream also, isn't it? But they don't run for president, or at least will have a significant record in politics. Trump has zero political experience, he never even attented a town council. (He also has zero experience in military issues or diplomacy.)
Trump will treat everyone beneath him as dirt, and per #1 that will be everybody.
Trump has repeatedly banned news organizations and reporters from his events because they were critical towards him.
“Love him or hate him, Trump is a man who is certain about what he wants, and sets out to get it, no holds barred. Women find his power almost as much of a turn-on as his money.” This could be a quote from a political analyst, but it's Trump himself who said it. Also: “All of the women on The Apprentice flirted with me – consciously or unconsciously. That’s to be expected.”
“If I get my name in the paper, if people pay attention, that's what matters.”
The world owes Trump everything; he, in turn, owes it nothing.
Trump is infamous for not honoring commitments. He bankrupted his Atlantic City casinos, stiffing his contractors for hundreds of thousands of dollar, but at the same time made millions of it himself.
Trump kicks up a storm whenever anyone disagrees with him or criticizes him. Too many examples to even start listing them all . Just take a look at one of his speeches, it doesn't matter which one.
After Rosie O'Donnell commented on Trump's bankruptcies , he replied: “Rosie O'Donnell is disgusting, I mean both inside and out, Take a look at her, she's a slob. She talks like a truck driver.”
Everyone wants to be Trump. As he puts it, “The old rich may look down their noses at me, but I think they kiss my ass.”
Nobody matters but Trump. Trump offends all kinds of people and groups of people on a daily basis. For instance, he sneers at Golden Star parents who lost their son in Iraq. There are myriad examples.

Grandiosity

One characteristic which often occurs in a person with NPD is Grandiosity. Grandiosity is an unrealistic sense of superiority — a sustained view of oneself as better than others that causes the narcissist to view others with disdain or as inferior — as well as to a sense of uniqueness: the belief that few others have anything in common with oneself and that one can only be understood by a few or very special people. In Trump's case grandiosity often leads to ridiculous statements:

Trump: “I went to an Ivy League school. I’m very highly educated. I know words. I have the best words.”

So Donald Trump knows words. Well, well, who would have thought. And not just any word, he has the best! (Whichever they may be…)

And when asked on MSNBC’s “Morning Joe” who he talks with consistently about foreign policy, Trump shows that he doesn't understand the meaning of “consulting”:

Trump: “I’m speaking with myself, number one, because I have a very good brain and I’ve said a lot of things.”

Dunning-Kruger

The Dunning–Kruger effect is a cognitive bias in which low-ability individuals suffer from illusory superiority, mistakenly assessing their ability as much higher than it really is.

Trump at a rally: “I know more about ISIS than the generals do, believe me.”

Conclusion

Jeffrey Flier, professor at and former dean of Harvard Medical School sent this tweet about Trump's NPD:

Flier: “Narcissistic personality disorder. Trump doesn't just have it, he defines it.”