In the haftarah of Shabbos Shuvah, it states (Hosheia 14:2-3) ָּׁ ַלְתכָש כִּי אֱלֹקיךָ 'ה עַד יִשְׂרָאֵל שׁוּבָה 'ה אֶל וְש ׁוּבוּ דְּב ָרִים עִמָּכֶם ק ְ חוּ ,ָבַּע ֲוֹנֶך , "Return, Yisrael, to hashem your G-d, for you have stumbled in your iniquity. Take words with yourselves and return to Hashem." We see the words וְש ׁוּבוּ ...שׁוּב ָה, to return to Hashem twice in the pasuk. The Chasam Sofer (Drashos Shabbos HaGadol year 5555, 17a) explains that this is because there are two types of aveiros, למקום אדם בין and בין לחבירו אדם , sins one commits against Hashem, and the sins one commits against one's fellow man. The Torah is telling us to do teshuvah for both forms of aveiros. The pasuk begins with aveiros that are bein adam l'chaveiro, as it states, אֱלֹקיךָ 'ה עַד יִשְׂרָאֵל שׁוּבָה, and עד means בכלל עד ולא עד, which means until Hashem, and before it reaches Hashem. This refers to aveiros ben adam l'chaveiro, between people (see there more). Afterwards, it states 'ה אֶל וְש ׁוּבוּ דְּב ָרִים עִמָּכֶם ק ְ חוּ, which refers to doing teshuvah for aveiros ben adam l'makom, which one transgressed against Hashem.
On Yom Kippur, we want to attain atonement for all sins, both למקום אדם בין and לחבירו אדם בין. However, to attain atonement for sins לחבירו אדם בין, for one's fellow man, one must first ask forgiveness. As Chazal (Yoma 85b) say, "Aveiros ben adam l'chaveiro, Yom Kippur doesn't atone for, before he asks forgiveness from his fellow man." The source is from the pasuk (Vayikra 16:30) 'ה לִפ ְנֵי חַט ֹּאתֵיכֶם מ ִכֹּל אֶתְכֶם לְטַהֵר עֲל ֵיכֶם יְכַפ ֵּר ה ַזֶּה ב ַיּוֹם כִּי תִּטְה ָרוּ , "For on this day He shall affect atonement for you to cleanse you. Before Hashem, you shall be cleansed from all your aveiros." The atonement is for 'ה לִפ ְנֵי, the aveiros he committed against Hashem. For the aveiros of bein adam l'chaveiro, he has to ask their forgiveness first.
Therefore, Shulchan Aruch (606:1) states, שׁ ֶיְּפַיְּס ֶנּוּ עַד מְכַפ ֵּר ה ַכִּפ ּוּרִים יוֹם אֵין לַחֲב ֵרוֹ אָדָם שֶׁבֵּין עֲב ֵרוֹת, "Aveiros ben adam l'chaveiro, Yom Kippur doesn't atone for them until he appeases his friend."
People say, "Why should I ask mechilah? He is the one who did something wrong to me. He should ask me for forgiveness, and not me from him." The Sfas Emes (Yoma 87b) reveals to us that it doesn't make a difference who is right and who is wrong. When there is a rift, a machlokes, each person should feel responsible to make peace, even if that means asking forgiveness for something he didn't do, and he is totally innocent.
The Sfas Emes supports this lesson from the following Gemara (Yoma 87b): Rav was learning Torah in front of Rebbe. Then Bar Kapara came into the beis medresh. He missed the beginning of the shiur, so they started from the beginning of the lesson, once again, to accommodate Bar Kapara. Then Reb Shimon b'Rebbe came in, and once again, they went back to the beginning of the lesson. Then Rav Chanina came in. Rav said, "How many times are we going to go back?" Rav Chanina was insulted. Rav asked for forgiveness. For thirteen years, Rav went to Rav Chanina on erev Yom Kippur to plead for forgiveness for what he did, but Rav Chanina wasn’t mochel.
And he saw in a dream that Rav would become the rosh yeshiva. Rav Chanina feared that this meant he would die, and Rav would replace him. Therefore, he didn't forgive Rav. He wanted Rav should leave Eretz Yisrael and go to Bavel. This is what occurred. Rav became the rosh yeshiva in Bavel, and thereby, Rav Chanina stayed alive and continued serving as rosh yeshiva in Eretz Yisrael.
Shulchan Aruch (606:1) writes, "Even if one only harmed his fellow man with words, he must appease him and ask forgiveness." The Sfas Emes writes that Rav didn't do anything wrong. He had a right to say that he didn't want to begin at the beginning again. Nevertheless, for thirteen years, he tried to make peace with Rav Chanina, and he asked him for forgiveness. This is because one should ask forgiveness from his fellow man, even if he didn't do anything wrong.
The Zohar relates that Reb Abba once saw a man tired from his travels lie down on a mound of earth. A poisonous snake approached the man, but a moment before the snake struck, a heavy object fell and killed the snake. 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, "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, who were family members, and it is proper for family to forgive one another. But you forgive those who aren't your family, and this is a higher level. Therefore, you deserve that Hakadosh Baruch Hu should perform many miracles for you."
From here we see that forgiving one’s fellow man is mesugal for Hashem to perform miracles for him.
Forgiving others is also mesugal for good children. Chassidim say that the mekubal Reb Yaakov zt'l merited to be the father of Rebbe Aharon HaGadol of Karlin zt'l in the merit of forgiving others. There are two versions of the story. One version is that before Yom Kippur, Reb Yaakov was handing out machzorim. A wealthy man had to wait a few moments, with his hand outstretched, until he got his machzor and was very insulted. He therefore smacked Reb Yaakov across his face. According to a different version, Reb Yaakov was the chazan on Yom Kippur night and davened a very long Shemoneh Esrei. When he took three steps back, a wealthy person smacked him for davening so long.
Either way, the end of the story is the same according to both versions: Reb Yaakov didn't harbor any hard feelings. He immediately forgave the wealthy man. Those present asked Reb Yaakov to forgive the wealthy man. "He was probably having a bad day," they said. "He certainly regrets what he did. Forgive him." Reb Yaakov told them that he had already forgiven him. In that merit, a year later, his wife gave birth to Reb Aharon HaGadol of Karlin zy'a.
Mishnah Berurah (siman 606) teaches that when you forgive others, Hashem will forgive you. When you forgive your fellow man who harmed you intentionally, Hashem will forgive you for your aveiros that you committed deliberately. "But if you don't forgive, Hashem won't forgive, either."
