You forgive your own bad days by explaining them. Give strangers the same option.

When you are short with someone you know exactly why, and the reason makes it understandable. When a stranger is short with you no reason arrives, so your brain supplies the simplest one, which is that they are just like that. Same behavior, two completely different explanations, and the only difference is how much you know. Everybody gets context for themselves and none for anyone else.

Next time somebody is rude to you, invent a plausible reason for it that has nothing to do with you. You will not know if it is right. You will notice how much lighter it is to carry.