2.1. The primary reason is dopamine
Dopamine (DA) is a neurotransmitter produced in the human brain, its production is localized in the hypothalamus. Dopamine is synthesized from the amino acid tyrosine through a subsequent step of L-dioxyphenylalanine. Then, noradrenaline can be produced from dopamine (also in the hypothalamus). Artificially synthesized dopamine, injected into the blood, acts as an activator of cardiovascular activity along with noradrenaline, but this hormone barely penetrates the central nervous system via blood due to the blood-brain barrier (this is also a scientific definition – author's note).
Dopamine is one of the chemical factors of internal reinforcement (FIR) and serves as an important part of the brain’s “reward system”. Dopamine induces feelings of pleasure (or satisfaction), which affects the processes of motivation and learning. Dopamine is naturally produced in large quantities during subjectively positive experiences such as sex, eating tasty food, and from pleasant bodily sensations. Neurobiological experiments have shown that even reward memories can increase dopamine levels, which is why this neurotransmitter is used by the brain to assess and motivate, reinforcing actions that are important for survival and procreation.