I think you're right. Obviously conditions like bi-polar or schizophrenia and some forms of depression originate in the chemical imbalance of some difference in the way the brain functions and regardless of outside stimuli.

At the same time, though, there is a reason behind most behavior, most decisions, most reactions. Depression is a good example because in some people that chemical imbalance that leads to depression happens "naturally" regardless of external factors, yet in many people it's also true that the chemical imbalance comes as a result of external stimuli.

Plus, the way things manifest, whether depression, anxiety or other things, the way we react to outside and to inner stimuli is influenced to some extent, great or not, by what is going on around us, by life experiences, by environment, etc, so they come as a reaction, an adaptation to those things.

There's a reason behind everything, the condition itself can be the reason and what provokes it, whether biological or external, is the reason why it exists.

Actually, if I can get a bit more philosophical, everything we call by one name or another, depression ,love, sadness, illness, is ultimately only a set of reactions to something happening either externally or internally. Everything is action and reaction all around us, there's nothing without a cause and even that cause has its own cause. That's also why everything you say or do in relation with other people and the world around you matters, because everything interacts with everything and has an affect.

A lot of mental issues are either triggered or aggravated or created by some sort of negative experience, traumatic events, deprivation of needs and so on. It's foolish not to acknowledge that.

For ex, I find it fascinating to look at serial killers, there's always a reason, always something more than psychopathy that got them on that path along with, in some cases, psychopathy.