Bentham's Axiom of human nature: We first and foremost strive to maximize pleasure and minimize pain.
Valuing: An act of bestowing positive emotion upon an entity, or simply feeling good about it.
Betham's axiom shows that our drives are first and foremost about what makes us feel good. A direct experience of pleasure always intensifies our emotions more than an indirect. In other words, we are made happier by eating chocolate ourselves rather than by hearing about somebody else eating chocolate.The same principle applies to all other pleasures. Altogether we tend to value our own happiness more than that of others.
Hence, as a result, it is very natural for us to value our own happiness more than that of our descendants.