The behaviour of people in terms of civic duty, ability to love and empathise, act altruistically and charitably, have compassion, behave ethically, and so on does not depend on a belief in God. These behaviours are intrinsic to humans. In fact atheists can feel love and compassion, act altruistically, charitably and ethically, empathise and so on. By many standards they can, and do, lead "good" and "virtuous" lives.
Perhaps the first consequence of denying the existence of God is the loss of immortality. A belief in God brings with it the possibility of everlasting life, and thus immortality. For an atheist, life ends when death occurs. But there is something very sobering on offer to a person who comes to accept his or her own mortality. The situation is analogous to a child growing up and facing the world at large. It may be daunting and difficult for a young adult to take on the challenges of living independently in an uncertain and demanding world. There are no great joyful and blissful rewards for the effort. The only reward is the independence itself. But this is a reward worth having because it allows the person to reach their full potential as an adult. In fact, not striving to achieve independence is surely a sign of maldevelopment. Coming to terms with mortality is very similar. The reward is simply the acceptance of the objective reality of death. It allows the person to put things in a new perspective. The perspective is not as extreme as when a terminally ill person prepares for imminent death, but it is something of that kind. In other words, it allows a kind of preparation for the death that will eventually come. Just as a young adult will feel contentment in achieving independence, an atheist can experience contentment in accepting mortality. Both cases represent milestones on the road to maturity.
Life without God is certainly not without meaning or purpose. On the contrary, an atheist can explore the broad spectrum of human behaviour and find new values in people. Rather than being painted in the black and white colours of good verses evil, with no shades of grey, the world is a richer tapestry of diverse perspectives and values. In place of religious purpose is the purpose of exploring and understanding the intricacies of this richer tapestry.