The thing is though, there is lots of evidence linking various mental illnesses to genetic factors and imbalances of various neurotransmitters. I believe there is evidence that both can come to play in bipolar- hence drug based treatment often working for people.
Often, for all sorts of mental illness, stress can be a trigger, or trigger the illness to get worse, however, I think it is wrong to say environmental factors necessarily cause mental illness, although they may influence the behaviour of an individual.
One example of this might be anorexia, and other eating disorders- there are obvious cultural and social factors that influence sufferers. However, there is also high co-morbidity between anorexia and other mental illnesses, such as anxiety and depression, which suggests there is more going on than just the social factors.
There is also a link between socio-economic status and mental illness, or rather there is a correlation between low socio-economic status and some kinds of mental illness such as bipolar and schizophrenia. Some people have suggested that this link could be due to the increased stress people of low socio-economic class face. Others suggest that these illnesses make it harder to have a successful career in many industries, so it is the illness causing the low socio-economic class. There could also be other, more complex explanations. Of course, these theories are very hard to accurately test.
I do agree that sometimes normal human emotions can be over-medicalised. However, I think that in most cases, people do benefit from help and treatment, if they can access it, and in many cases having a "label" helps people access appropriate services. I also think the social model of mental illness is ultimately flawed in many ways- I think society may influence how people's problems express themselves, but the majority of the time, there is some kind of cause at a chemical/genetic level as well.