Reference:
Everyone has personality adaptations in order to survive and socialise. Some become more dysfunctional and extreme than others and find it difficult to cope in the world = mental illness.
According to the Psychological (biological) perspective - Humans (and non-human animals) inherit the Genes coded with; abilities, characteristics or behaviours that enable
survival and
sociability within their environment - through what's termed as 'Natural Selection'.
This process then leads to 'Adaptations' - an evolved function, which accommodates immediate influences from the environment/culture (chemically and socially).
Personalities clearly differ, by their ascribed traits - i.e reliable or fun-loving to unreliable anti-sociable etc. However, most people suffer from a state of depression - persistent or short-lived and, whether it's relatively mild or extreme - it's deemed as a mental illness.