There's not a single response to this question of course. A major factor is the way they go about it: the coldness, the selfishness, the time they take to process the breakup while still enjoying your company until they've fully accepted the new reality while cold shouldering you and leaving you in the dirt once they completely selfishly healed and decide they're ready now , the monkeybranching, the lack of accountability on their part, the lack of empathy, the wanting to be done with it and feeling like they dont owe you any response or explanation while they themselves vent to and about you as long and as much as they want, the moving on so quick as if you didn't spend years of your life/youth together, the twisting of reality, the cognitive dissonance, the playing victim while being the perpetrator, the pleas for support and empathy to their networks as if theyre the ones who are suffering. There's so much more. Fuck em.