How many times have you had a negative interaction with someone or have gotten to know someone well enough and after getting to know them you think to yourself "man, Jerry really sucks!"?
Probably too many times to count.
You're left irritated, angry, and fed up.
I get it. You can allow yourself to feel that way, or you can dig a bit deeper to seek understanding as to why that person is the way they are.
It's not to excuse their behavior, but it does help their behavior make more sense which can help you with letting things go.
For example, I had an older woman flip me the double bird in a parking lot recently. Not just one middle finger, but two! I honestly have no idea what I did to irritate her, as she was driving toward me and we came no closer to one another than simply passing one another, but after laughing it off I thought to myself, "how bad of a day must she be having that she felt the need to not deliver just one middle finger, but a hands off the wheel, the car is still moving, double bird with a fist shake?" I'll never forget it because it was hilarious to me, but damn, she must have had a rough day up to that point.
A long-term example would be a life-long "friend" of mine. I use "friend" because I still care about him and wish him well, but as we grew older I realized how different we were as human beings. Our values are not the same and he can be a complete asshole, but I see that the apple doesn't fall far from the tree. It's no wonder he turned out the way that he did. It's not an excuse for his behavior and how he treats people, but it helps me make sense of "that's why he is the way he is". For years now I've kept my distance, and when I hear of his ways, I don't let it affect me.
Just remember, when you see someone actin' a fool chances are that something bad happened to them either today or long ago. They weren't able to bounce back from it and now you're left to deal with it. Hopefully, you can now figure out a better way to handle it rather than getting irritated, angry, or fed up.