Empathy. People criticizing each other often make the same "mistakes" that are nothing but normal human behaviour. Once you understand that we are all pretty much equal, you start realizing that most bad things are sistematic. There are few bad people, most people are quite nice and forced (or taught) to behave badly
Add on top of this “maybe you don’t need to worry about criticizing others in the first place” and you’re well on your way to a happier existence.
Disclaimer: of course thinking critically is important, and there are areas where you’d be irresponsible not to be critical of others. I’m talking about the IV drip of negativity of constantly getting annoyed at things that don’t affect you.