Atheists find it very unlikely that a God exists. Many of us strive to be "good" people for personal reasons. Rather than being moral simply because we fear retribution if we are not, we draw the strength to do the right thing from ourselves alone. An Atheist is accountable to nobody but him/herself and may do as he/she wishes. Which is precisely the reason we try all the harder to rise above base, selfish human nature and be more than we are. To fall prey to selfish urges would be, in our eyes, a sign of weakness of character. Atheists follow no written code of conduct, but develop their own personal sense of honour and code of ethics. We would rather ask questions, make mistakes and learn from them than blindly follow someone else's rules. Being for us is becoming. We can't blame a devil for our transgressions, and we can't shrug off failures. We don't pray for help, we help ourselves. When we screw up, we get up again, brush ourselves off, and try harder. What makes many of us aspire to be "good" people is the knowledge that we only live once. We are therefore driven to live well, and avoid causing others unwarranted pain on their journeys through life. When we help others in need, we do so out of compassion for them and not out of fear of punishment for neglect.
A man can have all his material posessions, his family and even his life itself taken from him, but nothing can ever take away his identity and pride. We want to be proud of who we are. We want to be strong and altruistic because it is too easy to be weak and selfish. We want to enrich the lives of those around us.
All this so that when the end comes, and knowing there will be no life after death, we can look back and say "I lived well. I was strong where others were weak. I was a steadfast friend and/or a caring lover and/or a good father/mother. I did it my way, and I made it on my own steam. I have made at least one other life better because I lived."
After all, if a god did exist and this god would condemn us for the trivial crimes of refusing to follow and refusing to worship, then that god would be a tyrant. I'd rather burn in hell.