Living organisms require for their sustenance a frequent supply of nutrients and water. They also require a habitat. For the survival of their species they must reproduce.

Individual organisms belonging to a higher scale of evolutionary complexity will sometimes find themselves facing situations that demand a fight-or-flight response. It is a question of immediate survival, of protecting their personal integrity. Aggressiveness is the key to fighting for immediate survival.

In humans, aggressiveness plays again a role in a second type of --exclusively-- fight response, this time related to long-term survival, when the threat is not to the individual's body but to basic resources of sustenance, habitation, and reproduction

Therefore, If evolution determined that 'badness' is the motor for survival and advancement of organisms (at the expense of others), in humans, aggressiveness is doubly needed for the avoidance of becoming a source for the advancement of others. However, the human mind --as a result of several possible cultural or just pathological mechanisms-- blurs the understanding of aggressiveness for long-term survival. The resulting acts by the human being may verge on evil.

It has been assumed that the organism feels hunger or sexual urge as a response to appropriate biological signals reaching specific neural centers. These centers were supposed to make conscious the need to satisfy the corresponding necessities for survival of the individual and for the fulfillment of his reproduction instinct.
Such a simple mechanism was recently made obsolete, when it was experimentally shown that the neurotransmitter dopamine plays an intermediate role in survival-oriented neural messages.
The biological signal generated by the --as yet absolutely unconscious-- need for alimentation or for the expression of a reproduction-oriented sexual act, arrive at their specific primary neural centers, where they stimulate the release of dopamine. This chemical subsequently acts on a second center, which in turn gives rise to the conscious sensations of hunger or of sexual arousal.
Putting it in graphic language :

1.- "The body needs food."
2.- DOPAMINE is released.
3.- YOU FEEL HUNGRY."

The previous scheme allows the understanding of several well-known but yet unexplained observations, as follows.
Anorexia not explained by physical illness might be caused by a derangement of dopamine release, due to a genetic-psychological abnormality.
The appetizer effect of alcohol in low doses is probably related to the stimulation of dopamine release. A similar effect is obtained with low doses of cocaine.
Increasing amounts of alcohol in the brain might cause excessive dopamine release, with resulting unwarranted feelings of sympathy toward others, followed by aggressiveness and subsequently by loss of food and sexual-oriented desires, as happens with cocaine intake.

It is probable that sympathy, love and aggressiveness are also modulated by dopamine, which might also be the target of aromatic sensorial stimuli that enhance food and sexual appetites.