Haha. I'm an atheist. My heathen, devil-inspired opinion follows:
1. A system of moral laws from a supposed deity is just as arbitrary as any system. If the Christian Church can make a set of moral laws, so can I...and both are equally as valid so long as they are supported by justifiable reasoning.
2. Don't you find it odd that the reason that you are "good" is because someone told you to, with fear of punishment (Hell) for not following them? Sounds like an angry parent dictating rules to a child to me. I'm "good" because I decide to be on my own. It's liberating. You should try it.
3. Social relativism: If I didn't have some standard of moral that is at least roughly consistent with the status quo, I'd end up in jail or dead. Same with anyone, religious or not. Why do think religious teachings, on the whole, have mellowed through the centuries? It's so they aren't persecuted by the general populace for being too rigid and evil. Witch burnings ftw!
4. Evolution. Inamilist summarized it well. basically, our bodies are programmed via thousands/millions of years of genetic conditioning to be altruistic in a general sense toward the species (and more specfically toward close relatives) to ensure the survival of our genes. In an evolutionary sense, terms like morality, altruism, and selfishness are explained fully and the whole thing becomes de-mystified.