High levels of serotonin (a chemical which acts as a neurotransmitter in the human body) are associated with states of Mania. Mania is a disease in which patient have mental illness, there are many symptoms which shows that the person is a maniac patient. Serotonin, a chemical can effect the mood and behavior so that is the reason when that level is increased it effects on the person.
There are many different factors, but some are: the amount of natural resources (food, water, etc.), amount of predator/prey, and availability of shelter.
