Without God, there is no definitive reason for anything, especially for working your whole life. That being said, those who are too weak to guide themselves onto a path and a goal that they choose to live for, they usually turn to religion. Surely we do not need a definitive reason to live otherwise we should kill ourselves? I like to think of it this way, lets say we, in an alternate reality, are all soldiers in Gods army and we have just one an excellent victory against one of his many opponents. Because of this victory he has sent us on a 6 month holiday with very little money, so we have to work a bit, but we try to have as much fun as we can when we can. That there is life, don't work the whole time because you're on holiday, but you have to work a bit to survive.