There were some instructions that they Israelites lived by during the time of Moses that when Jesus came it took a new turn. One of them had to do with vengeance and forgiveness. When Moses was still with the Israelites the lived by the order of "an eye for an eye", "a tooth for a tooth." But when Jesus came it became different. In the case of Jesus, He told us when we are stroke on a cheek, instead of retaliating we should turn the other cheek.
Christ doesn't want us to hold anything against anyone and that can be only achieved through forgiveness. One of the signs that proves our maturity as Christians is our willingness to forgive even before we are asked for forgiveness. Jesus forgave all those who persecuted Him even though they didn't ask Him for His forgiveness
Mark 11:25 – And when ye stand praying, forgive, if ye have ought against any: that your Father also which is in heaven may forgive you your trespasses.
Unforgiveness can compromise our prayers even though it is offered to God. One thing that God looks at whenever we pray to Him is our hearts. Our hearts need to be void of any grudges or anything that stands in the way of our prayers being answered. The Word of God does not encourage us to keep malice. Offences are bound to come but despite this fact we should learn to forgive at all times.