i've been wondering about this because atheists generally tend to get pissed off when religious people say that an atheist can't have morals because they don't believe in god. as an agnostic, i've tried to think of why i should have any adherence to some morality, you know, in case there's no god. so first i need a basis of morality, and second i need a reason to follow it.
if i assume there is no god, i don't want to permit some supernatural laws of good and evil here, because imo thats not much different from the idea of god. stuff like karma might as well be an automated vending machine god that judges you and deals your rewards and punishments. in the euthyphro, plato tries to establish that good and evil are above the gods, but imo again if it matters whether you adhere to these supernatural laws (if doing good or evil has an actual effect on you at some point during or after life), i think that's pretty close to the idea of god in a different, less personalized form.
so the only place i could think to look to is human nature. as in, maybe morality is derived from survival of the individual and communities. maybe its evolutionary. i mean, it makes sense that murder would be bad, because it doesn't help the community survive. you can explore all of that and wonder why certain things should be bad or good in consideration of this, but eventually you have to address part two of this problem.
why should i adhere to this morality? as a human, am i obligated to increase the survivability of humanity? why? nothing really adverse happens to me if i don't always do that. sometimes, i could benefit more from "bad" things. this might be a shortsighted view, i mean, i don't know the exact implications economically of stealing sunglasses. but in this model, if you know youre not going to get caught, and maybe you know they won't even notice, why not do it? you've got some moral code, but there are scenarios where theres no negative effects for breaking it.