Because some people are more anxious or sensitive or going through a stressful time etc they may be upset by things the other party would shrug off. Because the other party then refuses to apologise or try and change that behaviour perhaps the hurt party feels it is deliberate
I agree that may well be the case in some circumstances Violet. Within the context of societal changes as I talked about earlier then the chances of what you describe are probably increased.
I also think that "upset" is sometimes used as a lever to create a desired outcome for whatever reason.
In the case of the example mentioned earlier where estrangement has happened or is threatened because childcare is not offered, it doesn't seem to apply, for example.
Such a wide range of possibilities for cause and effect sadly. 
Is it possible to remove a topic from "I'm on"
Terrible relationship with DIL - am I the problem?


. This is such an emotive and painful issue for many isn't it and the reasons for estrangement are many and often complex.
.