How does someone become mentally ill?

It varies and depends on the exact illness. Often they are born with it or born with a predisposition for it. Sometimes it is self-inflicted, like in the case of substance abuse. Substance abuse can, however, cause other mental illnesses. Then some people develop brain tumors or degenerative diseases of the brain. Some may suffer an imbalance in neurotransmitters. Then of course, there is severe emotional trauma. That can result in PTSD, phobias, and other stress disorders.
It varies and depends on the exact illness. Often they are born with it or born with a predisposition for it. Sometimes it is self-inflicted, like in the case of substance abuse. Substance abuse can, however, cause other mental illnesses. Then some people develop brain tumors or degenerative diseases of the brain. Some may suffer an imbalance in neurotransmitters. Then of course, there is severe emotional trauma. That can result in PTSD, phobias, and other stress disorders.