1. People are inherently cruel. They enjoy inflicting cruelty on others. It makes them feel good to be cruel to others.
2. People are self-deluding and believe themselves good. It makes them feel good to think themselves good.
3. These two things are hard to reconcile, but not impossible, because people are also good at making up Reasons to justify doing bad things and even praise themselves for doing those bad things. As they say, you are the easiest person in the world for you to fool. You want to be fooled, after all. And why would you have reason to lie to yourself? As far as you know, you're the most reliable, honest person in the world, and you've never given yourself any reason to doubt your word before.
4. The Reason usually offered to reconcile points one and two and is that there is a Higher Obligation to Something or Other which justifies cruelty on a (self-serving) philosophical, quasi-religious or actually religious basis. People tell themselves they are doing good by doing evil, and then this lets them feel both good about being "good people" and also lets them feel good about being cruel to innocuous nobodies. Not only can you do tremendously cruel things which would normally be completely unjustifiable and cause you to feel slightly bad for not being as good of a good person as you previously deluded yourself, but you can actually feel even better about yourself being an even better person than you previously thought, because you have the Bravery and Integrity to say cruel things which need saying, which many of the fussy-pants polite people just are too cowardly or too stupid to say.
5. The better someone tells me he is, the more certain I become that he is a monster. (Also: The tougher someone attempts to present himself on social media, the more confident I am that he is a physical and emotional weakling.)
6. People frequently insist to me how good and noble they are. The entire purpose of Twitter sometimes seems to be to provide a forum in which committed sociopaths can tell the world about their enormous hearts and pure spirits and noble intentions and grand ideals.
7. People in general are monsters and I have very little use for them.
Actually... Just to extend this: I have long believed that people tell you exactly who they are, right up front, if you bother to listen. I noticed some time ago that all the people I'd had failed relationships with told me within the first hour why the relationship would fail -- but I of course wasn't listening. I'm not saying they were bad people. I'm just saying they were pretty upfront about the reasons we were a bad match. I just didn't pick up on it.
I noticed a long time ago that anyone who claims to be "witty" in their Twitter profile is never witty, and is usually pretty dull.
When people describe themselves, they are not describing themselves. They are describing the self they aspire to be, what they aspire to be generally points out what they are currently failing to be.
It does seem to me a general rule that whatever someone bangs on about the loudest as a putative reason to recommend him is almost certainly actually telling you about his deepest insecurities and darkest failings.
In the comments, Notsothoreau writes, "It's just virtue signaling, as in "I am virtuous because I vote for people with a D after their name". You are somehow virtuous these days if you go on Amazon and bad mouthed Ivanka's perfume (which you didn't buy and have never smelled).
There was a time when people were virtuous boycotting grapes. Now they are virtuous by supporting the illegals that put Cesar Chavez's unionized pickers out of business. And the worst is saved up when someone they don't like dies."