Or am I just exposed to self-selecting individuals?
What on a recovery board?? Yes I would think that's likely but there's tons of research out there using randomised groups etc. My advice would be to stop looking at one type or one cause, or even one category for causes. However it's only based on my own hunches so take it with a pinch of salt!
Whether any of us can say we have perfect mental health is debatable, whether we can say that about any addict is debatable. I think the difference between general terms of health and illness surround the impact on someones life. We are ill when we get a cold but it passes with few life consequences, I think menatal health (being physical) is the same, we can have 'bugs' which pass or persistant health problems. GOOD diagnosis doesn't look to label UNLESS a life is being hindered and damaged and then there's a complex system that comes into play where things like addiction are taken into account.
What I'm trying to say is that non addicts will have brief periods 'bugs' of reduced mental health, addicts who are abusing themselves physically and mentally will likely have more but not ALL will have persistant problems.