You learn a lot about this stuff if you are a cashier. A lot of the time, people will say unpleasant things for no reason, apparently without meaning to be unpleasant. A good example is when they will complain, often rather nastily, to the cashier about the high price of an item. Common sense and logic would indicate that a late teens/early twenties scraggly-looking cashier such as myself is probably not responsible for high prices or able to do anything about them. However, these people either lack the ability to discern this, or the ability to care about being rude for no reason.
(A fun fact: when I was younger, I found it almost impossible to buy anything because of anxiety that I would annoy or offend service people with my facial expressions, eye gaze, and general slowness, or by taking too long to put change back in my wallet.)
At the school dining hall where I work now, there is no cash payment, but there many other occasions to display a lack of mindfulness. Until recently, most people used take-out containers, and I was required to ask them to open their containers. (I'm not some kind of hall monitor asshole, but my manager hates me and would lurk about in the hopes of being able to catch me not looking in a take-out box.)
There were three kinds of people. Some people remembered the rule and would open their boxes before I even had a chance to ask. Some people required a vague prompt (I would gesture at their take-out box and mumble something; it didn't really matter what I said because they picked up on what I was asking them to do). A third group of people, thankfully a minority, didn't understand the prompt and I had to do a lot of word-finding and enunciating to explain what I was asking them to do. (Believe it or not, cashiering is a mostly nonverbal job except for like one script that you use over and over.)
I didn't resent the third group of people--maybe they didn't eat at the dining hall much--but I did feel very affectionate towards people who needed little prompting or no prompts at all. I developed a huge crush on a particular girl who would come to the dining hall to get take-out every day by herself. She would always come up to the register, open her take-out box without me saying anything, and give me this kind of grimace-y, rehearsed-looking (but adorable) smile. The smile made her look sort of scared, but I didn't know if she was actually scared. I just thought that her whole scripted routine was the dreamiest thing ever.
I have seen this girl walking around with other people, so I don't think she is like a social outcast or anything, but she's obviously a bit shy or strange or something and I can see that some normal people might think she is "sketchy" or "off" if she smiles at them that way. But in my limited interactions with her, she has always been observant and done everything possible to make things easier for me, even though she doesn't know me. This makes me think she's great.