Well, let's start with the most basic and work our way up.
Reason #1: (Sociopathic self-preservation) Act without any moral standards or concern for others and pretty soon you'll either get shot or arrested. Even if you've got no morals whatsoever, you'd be a damn fool not to do this just as a Pavlovian response.
Reason #2: (Empathy) Cooperative societies with members able to empathise with and help each other are stronger than those without; for this reason, evolution has given most of us mental processes such as empathy and compassion, both in inherited genetics and social constructs that are passed on by nurture and education (often buried in some stupid religions, unfortunately). Of course genetics and upbringing cannot always be relied upon, so you might not have this capacity, in which case we move on to number 3:
Reason #3: (Rationalism) Recognise consciously that society is stronger and works better when people keep some basic standards, which happen to strongly resemble those moral teachings of most religions that aren't directly related to their god but get put in the relevant book anyway, because they're sound moral sense. Recognise that a functioning society is more pleasant to live in for everyone, and thus beneficial to yourself. Recognise that in a functioning society you are more able to realise your rights and goals than alone. Recognise that it's just much easier to be happy when everyone else is.
You might even be struck by the notion that a proper, working society might eventually come to be a "super-organism" composed of individuals in much the same way as you are composed of cells, and that by being part of it you become a component of the nearest thing to "god" that actually exists, ie the most advanced and powerful sentient entity that can possibly exist. Atheists, contrary to the popular slur, don't believe themselves to be god. Some of them, however, might hope one day to become a part of one, or have a hand in its creation.
