
The only reply I could think of that athiests should use in relation to AKK's question is that by performing a moral or ethical act (in general terms), that person gets some sort of
internal feedback - from their own body (blushing/ butterflies in your stomach etc). This ''good'' feeling acts as a form of reinforcement for that person and thus the act is continued/repeated. The act usually carries a double-reward in the form of the other person(s) saying thank you or something similar. So you have an internal and external reward system that encourages you to perform the act again (that's why you don't see someone commit suicide immediately after saving someone elses life)
All humans have that mechanism built into them, probably as a means of survival (you save someone from the jaws of a sabertooth tiger, you feel good about it and that person gives you a hot dinner, you just got two rewards and instintively your mind is registers that saving people from the jaws of a sabertooth tiger is beneficial to you, so do it again.) Critics would argue that is a considerably cynical mechanism as it is more to do with self preservation than helping others (altruism), but I'd counter with: ''next time your in deep trouble and you're crying out for help, I'll turn a blind eye''. That should shut them up. Although, the more polite response is: ''sorry, it's in my nature''
Now, the only difference between a theist and athiest in this regard is that with thiests we have an additional (and arguably better, overall) form of encouragement through the religious teachings. So, whilst athiests have 2 streams of encouragement, thiests have 3 - which is what I was trying to get at in my last, lengthy post on this thread.