How? Why?

Oh, God, how could I have done that?
Because you are a dirty rotten sinner, that's why you can do it!
Building the Bride to Forgiveness (Part 2) - Gary Chapman*

Have you asked yourself this question? Have you asked this question and replaced "I" with "he" or "she"?
I have asked both questions. 

For all have sinned, and come short of the glory of God
Romans 3:23

I can't remember who or where, but recently I saw someone write that we are not sinners because we sin, we sin because we are sinners. Forgiveness is essential. First from God, second toward others. 

For the wages of sin is death;
but the gift of God is eternal life through Jesus Christ our Lord.
Romans 6:23

Forgiveness from God is essential so that we don't spend eternity in hell. Jesus Christ secured forgiveness for us by dying on the cross for the sins of the world. 1 John 1:7b says the blood of Jesus Christ his son cleanseth us from all sin. His death covers our past, present and future sins.

If we confess our sins, he is faithful and just to forgive us our sins,
and to cleanse us from all unrighteousness.
1 John 1:9

Forgiveness toward others is essential for two reasons: one, because God has forgiven us and two, so we are not susceptible to the poison of unforgiveness.  To receive forgiveness instead of what we deserve is an awesome display of God's grace and mercy toward us. It's just wrong to freely receive that from God and not graciously give it to others. I am not at all saying that it won't be hard to do in some instances. There will be times when only the strength of the Holy Spirit inside of you is what prompts and allows you to forgive. Believe me, I know. Unforgiveness provides an opportunity for the devil to gain a foothold in our lives. It allows him and his demons to continually harass us. It leaves us open to bitterness and unhappiness which, if allowed to linger, can end up causing more damage to us than the original sin. One of my favorite quotes, which I've mentioned before, says:
Unforgiveness is like drinking poison and expecting the other person to die.

The grudge you have is not worth it. It isn't. Instead trust God and His word.

Dearly beloved, avenge not yourselves, but rather give place unto wrath: for it is written, Vengeance is mine; I will repay, saith the Lord. Therefore if thine enemy hunger, feed him; if he thirst, give him drink: for in so doing thou shalt heap coals of fire on his head. Be not overcome of evil, but overcome evil with good. 
Romans 12:19-21

*Gary Chapman's message comes in 2 parts. There is a link to part 2 above. Click here for part 1. It can also be downloaded in iTunes via the Focus on the Family Daily Broadcast podcast.