Being able to forgive others who have offended you in some one is very important.  In many schools of spiritual development, it is one of the first lessons you have to learn.  There are a number of reasons why.
    If you can't forgive another person, especially if you continue to hold on to anger about it, you are keeping yourself linked to that person in a negative way.  This negative connection hurts you as much, if not more, than the person you are angry with, and prevents you from moving forward into new situations and more positive relationships.
    Holding resentment against another person saps you of vital energy.  Energy that you need if you won't to advance spirituality.  A ship can't sail across the ocean if it is going to insist on staying anchored.  The anchor has to be lifted before the ship can move forward.  We can't move forward unless we release those anchors of anger and resentment chaining us to the past.
    If we refuse to forgive those who have hurt us or offended us in some way, than we should not expect others to be able to forgive us when we offend them, even if the offense is unintentional.  When we learn to forgive others, we drop a barrier around ourselfs that prevents others from being forgiving and tolerant toward us.
    Most of all, as I have mentioned several times before, we are all connected, we are all part of the One.  When we offend others we are hurting ourselfs.  When we refuse to forgive, we prevent out wounds; emotional, mental, and psychic, from healing.