Mathew 6:14

If you forgive men their trespasses, your heavenly Father will also forgive you.  

Forgiveness is vital to God.  God does not hold a grudge against us for our past sins, and He does not want us to hold anything against anyone else.  When we do not forgive, it makes it harder for us to receive from God.  Our unforgiveness becomes a block in our relationship with God.