Why should I forgive my neighbor after God has forgiven me?
Because God graciously forgave you in Christ while you were still a condemned sinner, you should forgive your neighbor freely. You are not forgiving in order to be forgiven; you recognize you are already forgiven. Since your neighbor has not sinned against you as you had against God, grace compels you to forgive.
Les explains that the crux of the matter is that if God was gracious enough to forgive me right exactly where I was as a sinner condemned, then who am I to say that I can't forgive the other person? Instead of grudgingly saying, "yes I forgive you so that I can be forgiven," the believer now recognizes, "I am forgiven." If Christ could forgive me who was most unworthy, only because of His grace, then I can certainly forgive that fellow, because he hasn't done against me like I had done against God. Paul writes to the Believer in Colossians 2:13, "...and you being dead in your sins, and the uncircumcision of your flesh Gentiles, he hath quickened together with him, having forgiven you all your trespasses." Les then asks, if God could do that for me, can I do any less for my neighbor? That is the teaching, and it is the whole difference between Law and Grace.