I agree about black and white thinkers. Many people are very simplistic in their thinking and really struggle with empathy or understanding any point of view that is not their own.
The list of things that annoys some people and sticks in their minds can be extremely long; a tone taken by someone that they didn't like, a look given which they felt wasn't totally neutral or expressed an opinion they didn't like, minor omissions of invitation or speech, any of the above with regard to a friend or family member.
Some people are 'at war' with the world....they expect people to be doing disagreeable things and are on the lookout for them constantly. They fuel the ideas and dwell on them. They seek validation of their feelings on forums or with others who feel aggrieved and stoke each other up in their annoyance. They want to feel cross. They want to consider what they could or should have said as a response and want to consider further actions/complaints. Letting things go doesn't come easily to them.
Of course,nthings happen which aren't little things. Big, terrible things happen and people do sometimes do truly awful things. Forgiveness as a process is really important in these issues. However, fortunately many of us don't experience these things. But pretty much all of are daily confronted with things which might not be considered 'ideal' in terms of tone, manners etc. The question is whether this a big deal to people or not.
Most people who are living healthy lives, briefly notice a rude tone, or look or minor selfishness, and move on and forget it within a few moments. It's noted briefly, sometimes with surprise and often with just acceptance that this stuff is widespread, and then it's forgotton in the bigger things of life that are happening and taking up thought.
The difference for some people is that this stuff takes up more headspace. It is acknowledged and dwelt on, rather than being instantly forgotton. Some people remember a throwaway look or comment which was considered rude for decades. Why is that? I think much comes down to self esteem, self worth and an ability to balance things effectively...to know when something matters and when it really doesn't. It's about having a sense of bigger things and priorities in life and not having so much void space so that these minor things fill that space and become important and grow.
Some people don't feel important or valued. They feel a need to prove themselves all the time and want 'respect' given to them, because they don't have it automatically through their own self worth. So a rude word feels serious. Someone showing a lack of respect for property, or manners, or behaving outside of expectations of rigidly defined norms, becomes a big deal. This rigid sense of the 'proper and only' way to behave or treat them, because of its rigida leads to frequent disappointments and a sense of a world which is wrong and the enemy. It can apply to friends and family and often applies too, to their in authority....annoyance and grudge holding against teachers that one had as a child or the teachers of your own children, GPs, nurses, those working in banks, council offices, in call centres....a sense that everyone is 'out to get them' and a constant sense of treated badly. It's debilitating and exhausting, and it's iften inter-generational....so those who struggle with this stuff pass it onto their kids too because that's what they model.
An ability to 'not sweat it' is really healthy.