The atheist starts from the given that God does not exist. Therefore, any belief in God is misguided on its face and needs supplementary motivation. Fear would probably be the number one reason for religion from this point of view, with a sense of belonging being another strong one.
The believer starts from the given that God does in fact exist, and so belief needs no supplementary motivation. Belief is a simple acknowledgement of the truth. It's the same as asking someone to explain the basis for their belief that grass is green. It's a nonstarter.