Marcus Aurelius opens Book 2 (On the River Gran, among the Quadi) of Meditations with this sentiment:

When you wake up in the morning, tell yourself: The people I deal with today will be meddling, ungrateful, arrogant, dishonest, jealous, and surly.

I understand where he’s coming from. But it’s a little too snooty and aggressive for my taste.

My go-to mental salve whenever I interact with someone being difficult:

• That person’s adverse reaction is rooted on a deep level of insecurity or fear.
• His environment cultivated / tolerated  such behavior.
• He doesn’t have the resources to know better.

No one is an inherently bad person. Nobody.