George Eliot had some strong things to say about motive, God, and artificial canals. I know of no God belief that does not include some significant form of afterlife belief, so I think that what Mr. Eliot said is also applicable to the afterlife:
Accessory motives may aid in producing an action, but they presuppose the weakness of the direct motive; and conversely, when the direct motive is strong, the action of accessory motives will be excluded. If then, the glory of God is to be ‘the absorbing and the influential aim’ in our thoughts and actions, this must tend to neutralize the human sympathies; the stream of feeling will be diverted from its natural current in order to feed an artificial canal.
-George Eliot, Evangelical Teaching, 1855
Lets take Mr. Eliot's concept and see how we can apply it to the afterlife:
The afterlife concept presupposes that this life is not our first priority. But when this life is our first priority, afterlife belief is discarded. So if the afterlife is to be the primary consideration in this life, it will neutralize our consideration for ourselves and each other in this life.
When this is the only life a person believes in, it becomes much more valuable, because it fills up the #1 priority slot that the afterlife would otherwise fill. It tends to make a person more moral and humane. Without God and afterlife belief, the happiness of you and your fellow man in this life is the primary motive.
Through the month of April, I have shown that the afterlife is unevidenced. I have shown that available evidence indicates that consciousness is purely material, and dependent upon a material brain. I have shown that people want their consciousness to continue to exist, and that they believe in the afterlife as a coping mechanism. I have shown that they start with the conclusion of their coping mechanism (there is an afterlife), and then provide emotional and fallacious reasons to hold that conclusion. And now I have shown that believing only in this life provides for a direct (and superior) primary motive: itself.