Asklemmy

There's a cognitive bias called the "fundamental attribution error".

Essentially, when we do something wrong we have an internal justification. If we cut someone off in traffic we have a good reason

"i'm late for work and i didn't mean to cut him off"

When someone else does it, they are an asshole. They cut people off because they have an inferior character.

Some people have more of an issue with this bias than others. Essentially, we all have an internal narrative. A certain "identity of self" that we believe in. For example, I like to think I'm a good person. So if I walk past a homeless guy and don't give a dollar of change I have in my pocket - it creates cognitive dissonance. I'm a good person - yet I didn't share my change with the needy.

These are two contradictory ideas and therefore something needs to be done by the psyche to bridge this gap. This is where self rationalization comes in.

"he's probably just looking to buy drugs anyways"

With the rationalization, we can keep our "good person" narrative intact and we move on with our life. This is a perfectly normal and healthy way of living. It's part of how humans learn to interact with reality and society at large.

The issue comes when we see actions done by others that we feel wrong us. We cannot see what is going on in their brain. We don't know what is going on in their life. Like Gandalf said

Many that live deserve death. Some that die deserve life. Can you give it to them, Frodo? Do not be too eager to deal out death in judgment. Even the very wise cannot see all ends.

When someone else does something, we do not have the same ability to rationalize away their actions. Therefore we are much more critical of them for an action that if we took, we would have justified away.

What does this mean? Don't hold a grudge. Try and give people grace and move on with your life. Maybe the manager is just an asshole - or maybe they believed, incorrectly or otherwise, that you were a net detriment to the team. Perhaps they were having a stressful time and they felt you made a mountain out of a molehill.

Perhaps if you were sitting in their position, you would have made the same decision to fire you. It's hard to see all ends.

tldr: forgive and forget, stop judging people so harshly

i've found people who are like this have an awful internal "mental space". always negative, everyone's always out to get them, etc. Life is much easier when you assume the opposite. Reality is what you make it.