Because helping others provides both psychological benefits and strengthens your social network. If one thing is evident about the human experience, it is that strong social networks ensure survival.
Thus, it is to everyone's personal advantage to create a strong social network, and this is done through generosity. Even the most tyrannical dictator needs to have an inner circle with whom he is generous. When he fails to appropriately reward his inner circle, the tyrant is deposed, assassinated.
This realization has made me far more generous than did my old fear of hell.
*really wide evil grin* You do realize that most of the planet disagrees with that for a good reason.
Most of the planet does not disagree with me at all. This morality I have described comes purely out of selfishness...and requires only selfishness...a trait all humans posses.
Go to the tribesmen of Afghanistan, the SS of Nazi Germany, the Romans, or even the Mongols, and you will see this trend. Those at the top are always the best-connected, and they established those connections through a series of favors.
Even with my own religious belief, the only thing that stops me from just going on some psychotic spree is the law.
No, the only thing preventing you from going on a psychotic spree is your fear of death. Even in a situation of pure anarchy, anyone you kill has family and friends. You know that those family and friends would likely seek revenge upon you, which would stack the odds against you in a fight.
So, you avoid that psychotic spree in self-preservation. This was true for humans long before the advent of laws.
While I agree that religion is not a requirement for morals, it helps a lot, since morality is subjective there needs to be something to compare it to. I see humanity as having overgrown, and like all things that overgrow it needs a culling, and my religious belief does not deny that killing humans is sometimes appropriate, but obeying the law is always important. Basically, the issue is extremely complex and to simplify it like this doesn't work well, it will always lead to pointless arguments.
Religion is subjective, and a particularly ornate form of law. Morality, including religion, all grew from human self-interest.