I would say it is when thoughts and feelings get in the way of living to the extent it becomes detrimental or dangerous.

That being said, as I have gone down this road in the past year, I was pretty shocked to see the various behaviors and actions that count as symptoms or whatever according to " the experts". Pretty much anything you do can be held against you as a sign you are mentally ill, IMHO, if you read books such as the DSM IV. When they told me I was bipolar, I found it kind of laughable, the various staff would keep saying " you're entitled to your feelings" and then tell me my feelings were wrong, messed up, illegitimate is maybe a good word. Then, I hear discussion of what should be called a mental illness, in the new DSM V for example, and some of it is a joke - seems to me they want to diagnose, label, treat, and stigmatize half of the population. $$$ couldn't have anything to do with it, could it? Nah.

I am still angry that they tried to tell me in essence " it's all in your head" or "you are manic" or whatever. I would like to see how some of these psychiatrists would react if they were threatened with being shot to death. I didn't handle it well, but that certainly don't mean I broke with reality at any point. They totally invalidated my feelings and reactions in a way that was very detrimental.