I was atheist for 7 years, 13-20. Believing in God was not a choice for me. Suddenly I knew there was a God. Please bare to understand, I was given the Holy Spirit. An extreme feeling a ecstasy fell over me (and with my past drug use I can tell you that is exactly how it feels). My choice was serving God, and experiencing this extreme love and esctasy, or not.
So for me, the answer for your question would be, I choose to do the will of God because of the strong feeling of love. That's one reason. "The Kingdom of God is not eating and drinking but peace and joy in the Holy Spirit." Luke 17:21"Jesus said, The Kingdom of God is within you!"
I understand why you are atheist, and the logic behind it. I went to a christian school for Junior High and was voted most Christ like, then soon after turned atheist. It is your choice, but the Holy Spirit is the greatest feeling in the universe. Because of it men have written so many different holy books, and even deal with persecution from man, even death.