For an atheist(more rather an agnostic), the very belief that his life is a result of the cumulative effects of chance happenings through antiquity, that he is extremely lucky to be a being, is reason enough to seek betterment of his life, and those of whom he cares of, so that he may enjoy for a longer while, what luck has befallen him.
To not know of what happens after his death, is reason enough to enjoy life when he is alive.
Your proposition seems to assume that existence is preferable to non-existence. What is the basis for that preference?
You talk about "those of whom he cares of"? Wouldn't a man of science recognized those feelings of "love" as nothing more than evolutionary mutations.
Also, what is "luck"? Is it satient?