A cleansing breath

No one gets along perfectly 24/7/365. No one. We get on each others' nerves, we've all got little ticks and habits and compulsions that drive other people crazy. What keeps us from killing each other or living lives of hermetic isolation? The need for other people, the recognition that we ourselves are flawed beings and maybe love.

No one can hurt us the way the people we're closest to, can. We learn each others' buttons, like there's buried treasure if we hit them in the right sequence.

Except...it's incredibly destructive. We bruise each other daily. In the most casual and unobtrusive ways. We've become a snarky, cynical society. Sarcasm is the forked tongue we speak from day and night. We expect our loved ones' to know when to stop. We expect that if you can dish it out, you can take it. What if these expectations are merely another flawed premise?

What happens when the lines are crossed and you don't know how to go back?
Person A is, by nature, a habitually snarky person. Usually harmless.
Person B is someone who isn't sure when it's a joke and when it's serious. Person B asks Person A to stop being snarky towards them. Person B apologizes, and stops. For a while. This becomes a pattern.
Finally, after Person B turns the tables on a remark Person A has made in the past, Person A makes remarks that are insulting to Person B. Person B attempts to illustrate this to Person A without being confrontational. This is ineffective. Person A repeats the insult. Person B is more direct and expresses their displeasure, indicating that Person A should try that behavior with someone else. Person A objects and replies to Person B in a form they've seen before. By using language that implies Person B is misinterpreting the exchange, Person A in effect, invalidates Person B's feelings. This is something that has happened before. Person B rejects this and indicates they are ignoring these messages and wishes to be left alone. Person A continues to send them. Person B then asserts publicly that Person A should stop contacting them. Person A then indicates they believe person B is  being both passive-aggressive and irrational. Person B merely desires an apology and some time free of contact with Person A in order to feel less hurt and angry.

Person A's continued attempts to communicate with Person B are ignored and erased, as messages begin to indicate Person A has doubts about Person B's mental stability. Ignoring them seems to be the best course, as Person B believes that they will be drawn further into a hurtful situation. Ultimately Person A asserts that they are not angry, therefore Person B should call them. Person B then feels additionally invalidated and upset.

How does this get resolved?
Can it be?
Why is the need for a cleansing breath and an apology cause to question someone's mental health?
Why isn't, "I'm sorry," the immediate response when someone feels hurt by something you've done?
If you know the answers, please tell me.