Since leaving Mormonism and religion in general (I am not an atheist however), the topic of morality has been one I've often thought about. Where does morality come from? Why should someone be moral? All questions eminently worth asking.
Back when I was a Mormon, I couldn't grasp the idea that a person could be ethical without a belief in the divine. I was very much a believer in Dostoevsky's idea that removing God from the moral equation would make it completely unsolveable.
I was wrong. God is not an integral variable because morality and ethics are actually fully natural phenomena that have developed over millenia. All animals who are gregarious engage in it to varying degrees.
Not stealing the other animal's piece of the kill is an act of morality within the community, as is leaving another member of the pack alone when it is sleeping. Protecting the pack from violent outside force is also an act of morality.
When you are a solitary animal, there is no morality. When there is no society, anything you can do is permissible since you cannot possibly offend or harm anyone who matters to you. Making someone you like sad or wasting someone's time are impossible when there are no other someones.
There are many advantages of being an amoral, solitary being. However, there are also many disadvantages. Two heads are usually better than one and two clubs fighting an animal trying to kill you are certainly better. That is why natural selection has favored those organisms who are social. Morality is merely the rule book that a particular society agrees to play by, both collectively and individually.
As time has gone by, morality has followed its evolutionary impulse. In other words, humankind has continued to develop more optimal forms of morality. This has led to greater greater altruism and better allocation of scarce resources. All of these things are what people generally term "progress." Others call it the "marketplace of ideas."
Once our lower order needs of safety, food, and reproduction were sufficiently men, eventually, human morality evolved to a point where ideas like universal humanity, ethics, compassion, and justice emerged. These ideas are non-religious although in most societies, their development was associated with the emergence of religion.
Ethically speaking, we as humans generally are operating on a paradigm of ethical increase. The longer we're around and the more advanced we become, the more we have turned our attention to refining what we consider to be moral. Examples of this process has been the emergence of secularism, equality for women, and the destigmatization of homosexuality. In terms of moral progress, some people are objectively more advanced than others just as some societies are similarly more evolved.
In the end, though, there is no absolute reason that you "must" be more moral than the minimum required to keep yourself out of jail; that is because we are all self-directed moral agents. Nonetheless, being what modern societies consider a "good person" has many advantages such as friendship, stability in life, love, self-esteem, sex, and reproduction to name a few. People who have trouble grasping or behaving morally always lack one or more of these things.
Then, of course, there is a certain value in moral behavior itself. To always examine one's life and to think often on what it means to do good can be very inspiring and uplifting.