Why is the person who was wronged always expected to be the "bigger person"?

Especially in romantic relationships where a person that was broken up with with no fault of their own and was heard and treated like trash by someone that they were once loved and as soon as that person decides that they no longer want to associate themselves with that person, the narrow automatically labeled the "bad guy"

As Petty as this may seem, I'll give an example. A person that had a relationship with someone that did not work out and was broken out with through (no fault of their own) due to the dumpers narcissistic selfish reasons and you decide to no longer associate yourself with that person and God forbid something bad happens to the dumper or they get themselves caught up in a very legal situation and you refused to help them due to the way that they've treated you and you've just decided to not care about them anymore and to completely ignore them, why is a seen as childish and Petty to no longer want anything to do with a person who have wronged them in any way.

Call me a narcissist all you want but if someone has dumped and hurt me through no fault of my own and decided that they don't care about my feelings at all Ade and zoned me thinking I should be fine with it, I wouldn't give a shit what happens to them not that I wish bad on anyone but I wouldn't care what that person goes through and I would not defend them if they were in any situation and I would not rescue them if they were harmed in any way I would not care about them anymore because after how they've treated me why would i? But it seems as though people have been brainwashed and conditioned to still care about people who mistreat and hurt them why is that?
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F. Y. I I do not believe in seeking vengeance against anyone I believe everyone should move on with their lives but I'm just saying why is it that the person who has been wrongfully broken up with due to the dumpers selfish reasons, is always expected to stick around and act as if nothing has ever happened is my point.
Why is the person who was wronged always expected to be the "bigger person"?
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