Besides what was already mentioned (and forgive me if I repeat myself), forgiveness:
- Sometimes makes you even forget what the person who offended you did in the first place.
- May not make you forget what the person who offended you did in the first place, but it may make you realize that what the person did in the first place is not worth holding a grudge over or actually even that bad. This doesn't mean that the person is right for what he or she did—in fact, he or she may be (at best) completely misguided in what he or she did (and may even continue to be doing), and even (at worst) deliberately wanting to be blind to what he or she did (and may even continue to be doing). Also, what did Jesus say? "Father, forgive them; for they know not what they do." (I don't have the reference right in front of me, by the way...it's Luke 23:34a. I was darned close—I thought that it was Luke 23:24 or something like that.).
- Make you the better person in that moment. "Take heed to yourselves. If your brother sins against you, rebuke him; and if he repents, forgive him. And if he sins against you seven times in a day, and seven times in a day returns to you, saying, ‘I repent,’ you shall forgive him.”" (Luke 17:3-5)
- Make you realize how much you need to be forgiven and hypocritically fall or fall back into unforgiveness. "Brethren, if a man is overtaken in any trespass, you who arespiritual restore such a one in a spirit of gentleness, considering yourself lest you also be tempted." (Galatians 6:1)
After all, Rabbi Jesus warned, "But I say to you that whoever is angry with his brother without a cause shall be in danger of the judgment. And whoever says to his brother, ‘Raca!’ shall be in danger of the council. But whoever says, ‘You fool!’ shall be in danger of hell fire. Therefore if you bring your gift to the altar, and there remember that your brother has something against you, leave your gift there before the altar, and go your way. First be reconciled to your brother, and then come and offer your gift. Agree with your adversary quickly, while you are on the way with him, lest your adversary deliver you to the judge, the judge hand you over to the officer, and you be thrown into prison." (Matthew 5:22-25)
Therefore, especially if you're Jewish like me—especially if you are Non Messianic and still under the Old Covenant—, for you to fast on Yom Kippur (which is on or around October 3, 2014, depending on which calendar you observe) would be quite hypocritical and pointless, for you will not be forgiven despite that you are שומר או שומרת הצום של כיפור.