humans are animal and as such we have the instinct to procreate. Maybe this is not true for all humans but most of them will feel that urge. Natural selection

humans are animal and as such we have the instinct to procreate. Maybe this is not true for all humans but most of them will feel that urge. Natural selection works pretty well on this: the one that don't feel that urge will be less likely to procreate and so to pass their genes to the next generation.