Shulchan Aruch (606:1) states, "Yom Kippur doesn't atone for aveiros ben adam l'chaveiro (that one committed against his fellow man) until he receives his fellow man's forgiveness. Even if he only harmed him with words, he must appease him and ask forgiveness. If your fellow man doesn't forgive you the first time you ask, go to him a second and third time."
Nevertheless, Shulchan Aruch adds that it is proper for one to forgive. "If someone asks you for forgiveness, don't be cruel..." You should forgive him.
The Mishnah Berurah adds that this is for your benefit because just as you forgive your fellow man, Hashem will forgive you. And when you forgive your fellow man who harmed you intentionally, Hashem will forgive you for the aveiros you knowingly committed. "But if you don't forgive, Hashem won't forgive, either."
The Zohar relates that Reb Abba once saw a man, tired from his travels, who lay down on a mound of earth. A poisonous snake approached the man, and a moment before the snake struck, a heavy object fell on the snake and killed it.
When the man awoke, he saw the dead snake and understood that a miracle had happened to him.
The man stood up, and the mound of earth he had laid on suddenly crumbled and fell down a cliff. He realized he was saved a second time. Had the mound crumbled a moment earlier, he would have toppled down the cliff together with it.
Reb Abba watched this from a distance and quickly came over and asked the man, "Please tell me your good deeds and the reason you merited these two miracles."
The man replied that it was because he forgives his fellow man. "At night, before I go to sleep, I forgive everyone who wronged me, and I seek to do kindness with them."
Reb Abba cried, saying, "Your deeds are greater than Yosef HaTzaddik's. Yosef forgave his brothers, and it is proper that family members should forgive one another. But you forgive even people who aren't your family, and that is a higher level. Therefore, you deserve that Hakadosh Baruch Hu should perform many miracles for you."
