You should forgive, no matter what.

Note the words of Jesus:

Pay attention to yourselves! If your brother sins, rebuke him, and if he repents, forgive him, and if he sins against you seven times in the day, and turns to you seven times, saying, 'I repent,' you must forgive him." (Luke 17:3-4) ESV

There are conditions for forgiveness; it isn't dispensed out-of-hand. There is (1) the sin or offense that was committed; (2) repentance on the offender's part; (3) forgiveness on your part. If someone enters your home, kills your wife, rapes your daughter and steals your X-Box, you don't say, "I forgive you." If, on the other hand, when he stands before the judge and seems remorseful, forgive the guy. That's biblical.

Forgiveness is also an extension of God's grace to us as we extend towards others. IOW, the Lord put up with a lot of nonsense from me; why should I not extend the same courtesy to my fellows? This is taught in Matthew 18:23ff.

Another aspect of forgiveness often involves offenses committed by others against you that they may not be aware of. Here's a simple picture: You're shopping in Walmart. You need help, so you corral an associate. The person not only doesn't return your smile or greeting, they don't give you much help. Now here are Jesus' words that can apply to this:

Whenever you stand praying, forgive, if you have anything against anyone, so that your Father also who is in heaven may forgive you your trespasses." (Mark 11:25)

You're praying in Sunday morning services and remember that rude associate. That person has forgotten all about you, but you haven't forgotten about him/her.  Keep in mind that they could have recently caught their mate in bed with a stranger. Or something equally distressing is going on. Nothing against you. Why should you hold it in your heart? Forgive them. I've found this to be a good prescription when dealing with mean, rude, or unhappy people.

On a side note, I worked for Walmart for 7 years and Lowe's for another 3. The retail industry is rife with customers who enjoy heaping abuse on employees. If a customer spoke to me like I was scum, I firmly but politely told them I didn't appreciate it. Now, how may I help you? Too many Christians adopt a turn-the-other-cheek mentality and snivel, "I forgive them." A Christian isn't on earth to throw his or her self-assertiveness around, but they aren't the neighborhood doormat, either.