Do people always find it hard to move forward from disinterest?

If someone who had a crush on another person, felt rejected by that person, why would they hold a grudge after 3 years? They did not ask the person out. But dropped “I love you” lines here and there. But this was not taken seriously. So this other person felt hurt. Buy even after they “moved” on to another person, who is a mutual acquaintance, they still behave hurt by the other person. They even go as far as joining someone else in teasing the person they said they loved. When the person is by themselves, they stare at this person. And if they try to talk to them, they reply annoyed or walk away. And only say things to hurt this person. Even after so long, and being in a new happy relationship, they treat the person they had a crush on like a stranger and enemy. Why would anyone do that?