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For political disagreements, be wary of fruitless endeavors. 20 replies back and forth are pointless. Most of the time, my goal isn’t to change that person’s mind; it’s to be the voice of disagreement so that others can either be exposed to my views and their rationalization or so that others who silently agree with me can see that these views aren’t unpopular. After enough time passes, I tend to state that I’m disengaging because it’s no longer going to catch the eye of many of these people.
In general, be courteous. Most people aren’t assholes. Some people will have a bad day and maybe will take it out in you, but gently asking them to be courteous and not take their bad day out on you usually spurs some introspection and improves the interaction. If it doesn’t, then they’re really not mature enough to further engage with. Respect yourself and don’t allow anybody excessive opportunity to ruin your day.
My spouse is a Starbucks barista. “You are special” goes on 90% of the cups she hands out. She puts actual effort into it for the people who are underappreciated workers like nurses and teachers.