The moral code

In a number of his works, C.S. Lewis writes about an inherent "moral code" that men seem to accept, giving them feelings like "that's not fair" when the moral code has been broken. I can't help but wonder, for people who do not have a relationship with God and who don't realize that our purpose, hope and joy in life is found through worshiping and honoring God, what reason do they have to follow any type of moral code? In other words, why should people be "good"?

My first answer to this question was: well it is necessary for people to follow some time of moral code so that they can have relationships with people and be accepted in society. After all, people who lie a lot and are prone to fits of rage and who take advantage of people do not usually make the best friends, lovers, employees etc. But really, different societies seem to have their own variation of the moral code. Gang members are praised for stealing things or murdering opposing gang members for the sake of their own gang. Other highly conservative societies might disapprove of dancing, girls wearing pants, or watching tv, when these things are not morally wrong (in my opinion).

Maybe peoples' consciences help to hold them to a moral law, the guilt of opposing the moral law causes them to obey it. In sunday school, i was always taught that if you did something wrong for long enough, your conscience got weaker and weaker until you were "numb" to your sin. Looking back, i dont know about the theology behind this idea, but it is true that sometimes the enjoyment of sin outweighs the guilt. Other times, the advantages of sin seem to outweigh the guilt, such as when a businessman "bluffs" to make a sale or get a promotion.

I'm sure there are other considerations... so I'll just ask you guys, why should people be good?