Because it feels good. That's how we operate, feels-good, feels-bad. Even an infant knows it.
ping
If their is a god then he has the power to enforce his view of morality. He can also violate the rules he makes because he has the power to do so. For example if you are an inerrant believer in the bible you have to accept the fact that the Lord ordered the isrealites to commit what we would call genocide.(Numbers 31)
Now I can say that from my personal beliefs this is immoral though against god I have no power to enforce that view.
If there is no god then whatever group of people have the might, whether it is through the sword, majority opinion, or other means, they make what is right.
As a side note I have always thought the deity that the muslims worship is more similar to the deity portrayed in the old testament then the deity in christianty is. For complete disclosure I am a deist.