So polio isn't an illness because anybody could get it in the right circumstances?
It been a while since I was in practice but the definition of a mental illness as I recall was having a set of issues/problems/symptoms that arise in the mind, are outside the control of the sufferer and disrupt the sufferers life or the lives of other people.
There are many people who hear voices or see visions and have integrated that into their daily lives without issue. Some people have a psychopathic character type and never do anything immoral or illegal.
I would call chronic depression an uncontrollable mood swing. I suspect that many of us have suicidal thoughts at some point in our lives, again its when the thoughts are uncontrollable and lead to life disruption. Addiction again is a sliding scale thing. I guess the line there is what the addiction does to the life of the addict and what happens when they can't get hold of what they are addicted too.