I don't like the "liberalism is a mental disorder" rhetoric, because it's divisive and creates an excuse to never listen to the other side.

The left has been putting derogatory labels on conservatives as a whole for years: Stupid, simple, evil, greedy, selfish, backward, religious nuts, etc.

Especially since Trump has been elected, I've seen tons of leftists basically say, "Anyone who supports a racist, sexist, awful man like that is evil, and is not worthy of being part of any conversation about politics", which in turn has greatly fueled the rapidly increasing hatred between the two sides.

One should never try to find a reason to dismiss the other political side as crazy/evil/stupid so you can justify not listening to their opinions. That will never lead to progress.

I don't believe liberals have a mental disorder. I do believe that some liberalism does sometimes spring from the need for certain people to alleviate guilt for being successful/comfortable. "I have so much and my life is good, but I see others suffering, and therefore I feel I don't deserve happiness. Oh wait, I bet if I vote Democrat and support left-wing causes, that means I'm helping the downtrodden! Okay! I just did something to alleviate the guilt. I really am a great person and do deserve happiness!"

The term is called slacktivism -- activism or support of "compassionate" political/social causes, where little effort or help is actually provided to the causes, and the main purpose is to make the person giving support to feel good about themselves.

Unfortunately, liberals have a hard time grasping that it is impossible for there to be complete equality in any society, and attempting to force that complete equality will result in all kinds of unintended negative side effects.