Many atheists ponder the question "Why do people follow these silly religions?" I think the answer is quite simple, really. Reality is oftentimes hard to deal with. The coping mechanisms that we develop as children usually involve a great deal of fantasy, or the complete blanking out of a bad memories with a fantasy cover-ups, which doesn't bug us until later in life. Many people outgrow these fantasies, and learn to accept the bitter parts about life. They learn to accept the fact that everyone will die, that bad people sometimes get away with horrible crimes and never go to jail, and that many of your good deeds will never be rewarded. It is my assertion that people who are religious do not want to grow up and accept life's negatives. They would rather deny the unpleasant realities of life, and cover it up with the patented fantasies that religions have to offer.
In a sense, nearly every religion serves the same functions. Even the polytheistic religions of ancient Greece and Rome provided comfort for the believer. A good Roman need not fear death, because in death, they can become Gods themselves (a common theme among many new age religions that survive today), and Norsemen would be carried away to Valhallah if they fought bravely and died with their swords.
Death is hard to deal with, even for non-theists. It's not pleasant for us to think about being dead, rotting away, being eaten by bugs. It's hard to imagine not living. We all have seen death in one form or another before, and the horrible ways that some people and animals end their lives is too horrifying for many of us to want to think about.
Religions comfort our feelings surrounding death. For our own deaths, religions usually offer a paradise or reward for making it through life. When somone you love dies, religions provide the reletives and friends of the deceased comfort in knowing that they will be "free" from life's troubles, "happy" forever, and that someday, when the rest of the family and friends die, they will all be reunited again. It's all an emotional mask for the horror and dread that many people have about death.
I think that the main difference between atheists and theists is that atheists do not let their mortality frighten them. In foxholes and on battlefields, the atheist is less likely to break down and pray to a god for divine help, and more likely to anylize the situation and try to find the way to safety that works for them. Some theists (not all... Many theists are rational and intelligent, and have courage greater than their peers) might be inclined to do irrational things (like pray for help or freak out and sacrifice themselves needlessly). Of course, I do not assert that all atheists are braver and more rational under stress than theists, nor am I implying that all theists are quivering lumps of irrational cowardice.
Likewise, the question of justice is another aspect of life that poses a serious problem for a theist. History is replete with tyrants who lived and died unpunished for their evil deeds. Many times, we do favors for people that never get repaid. Sometimes, good people die without ever getting a break from pain. This problem is often referred to as the question of ultimate justice by Christians. Evil people are taken care of after death by being sent to hell to suffer eternal torment for their wicked ways. As atheists, we know that this is just wishful thinking -- an almost chidlish fantasy that attempts to cover the fact that bad people often get away with evil deeds, and that good people often suffer without justice ever being done.
The Christian mindset is that everything will be sorted out by God in the afterlife -- all good people will get a special reward, and all bad people will get punished severely. The theist who believes in this kind of "ultimate justice" or post mortem justice, is really just undergoing wishful thinking. They cannot admit that sometimes, all the effort you put into justice amounts to nothing. They cannot deal with it, for they feel that unless everyone believes in this ultimate justice idea, that all will turn to chaos -- if people realize that evil deeds go unpunished ultimately, then that opens the door for all would-be tyrants and criminals to do as they please without fear of retribution.
We, as atheists, know that this is an unlikely case. We know that the very survival of individuals and groups depends on behaving in a manner that is not criminal or cruel. We know that people who do evil deeds oftentimes get punished by revenge-seekers, lynch mobs, police, and armies. Interacting with people depends on a certain degree of mutual trust, and if you do evil, or are known for being a liar or cheat, people are less likely to do business or favors for you. In a sense, I, as an atheist, believe that acting moral (reletive to the society that you are a part of) is a built-in survival technique. I believe that most people would behave in coopertive ways, even without police to keep us in line, because that is how we naturally are -- it is instinctual.
In Summary, the atheist accepts that death is a fact of life, that it's expected, and that there is nothing to fear from it. Justice is oftentimes eluded -- life can be unfair; good people suffer and bad people get away with making them suffer. The theist doesn't want to accept death for what it is, and has to comfort themself with a fantasy belief. Likewise, the theist doesn't want to accept the fact that justice is not always served, and that good people experience cruelty at the hands of evil doers, who live and die unpunished. They have to mask over the painful realities of life with a strong coating of candy, made from a fantasy-based belief system designed to comfort them from life's oftentimes brutal realities.