I have thought about this for sometime. Often my Christian friends will ask me "Why do you care?" so I want to try to answer that now. Not for every atheist because we don't all agree on much but only from my point of view.
First, I care about my friends. If you were being sold a bill of goods by a known con artist, I would not be a very good friend if I did not tell you what I know about the guy. In the end I can't make the decision for you but I do think that it is my responsibility as a friend to provide any information I have which might help you to make a decision.
Second, I care about my country. We have one of the highest crime rates in the Western world and in correlation to that we are the most religious! This country was found on principles of freedom. Europeans, sick of the stifling oppression of the church, sought a new world, a better way of life. Funny that, because now most of Europe is far ahead of us in the peaceful pursuit of happiness. They have lower numbers of religious people and lower crime.
Finally, I care about humanity. As we continue to evolve we learn more about what works to promote peace and well being for ourselves and our fellow humans. Humanity is not perfect and likely will not ever be. We continue to learn through trial and error, through our failures despite best intentions, and through the acquisition of knowledge.
My hope as an atheist is that the myths that have ruled much of humanity will be exposed and that each individual will understand that he is capable of making a difference in the world without superstition and fear.
We have only one lifetime and doing all we can to make it better for ourselves and those who come after is a worthy purpose. What could be better than to know that I have helped someone to think for herself, to give up on fairy tales and seek out knowledge without fear?