There are number of reasons why we try to stay away from sin. There is a world of difference between sinning in a momentary moral lapse and living an outright lifestyle of willful disobedience.
The first thing you have to ask yourself is, "who do I serve". Do you serve Christ or do you serve your flesh? This is a question all of us have to ask and, be careful, the answer often isn't as easy as you think.
Romans 6:6 tells us that "Knowing this, that our old man is crucified with Christ, that the body of sin might be destroyed, that henceforth we should not serve sin."
If this is true, then how can we who are dead to sin, live any longer therein?
Paul said (and I am paraphrasing here), "Now, if we know that where sin abounds, grace abounds even more, does that mean that we can keep on sinning because we know that God's grace is going to pick up the check? God forbid!"
Simply put, it's not our decision to make anymore. We surrendered that to God when we came to the cross.
If you're living a lifestyle of sin, then you're serving your flesh and serving sin, not Christ.
We abstain from sin for a number of reasons.
We try to abstain from sin out of respect for God and His authority in our lives.
We try to abstain from sin to honor Christ's sacrifice on the cross.
We try to abstain from sin out of respect for those around us who are weaker, spiritually.
We try to abstain from sin because sin hampers our relationship with God.
I'm sorry if this is a little rambling but I hope it helps.