There is little question that our greatest drives are to reproduce and to avoid death. Freud and Foucault argued that the two are fused, that the death instinct pervades sexual activity— a connection simply recognized in the French term la petit mort.
Freud believed that humans were driven by two conflicting central desires: the life drive (libido/Eros) and the death drive (Thanatos). Cathexis is defined as the process of the conscious investment of mental or emotional energy into a person, object or idea, and includes all creative, life-producing drives. The death drive, with the energy of anticathexis, represents the unconscious urge inherent in all living things to return to a state of calm: in other words, an inorganic or dead state.
On the one hand, the life drive promotes survival by avoiding extreme unpleasure and any threat to life: on the other, the death drive functions simultaneously toward extreme pleasure, which leads to death.
Physically we have the greatest resistance to harmful environmental factors during the period of our greatest reproductive capacity. Evolution has little interest in the survival of those who are in the post-reproductive period of life.