It's an important distinction that smart professionals realize that the public often fails to: a lot of mental illness diagnoses only happen when it's causing you distress.
Like, you might get a cold this winter. Maybe you get a runny nose and a sore throat. You deal with it. It doesn't ruin your life. Or maybe you have mild allergies. It's something that makes spring a little unpleasant, but whatever, you get some Allegra and deal with it. You don't go to a doctor and get a diagnoses and medicalize your sniffles or your irritation at being the unwilling participant in a floral bukkake event. You only look for help when it's bad enough to make living your normal life a frickin' struggle.
That's the difference between have a bit of the sads every once in a while, and being truly, medically, depressed. Or the difference between being a little geeky and having full-blown Asperger's. Or the difference between being bored at school and having ADHD. One doesn't really cause you pain and suffering in life. The other does. Just because the mild version exists doesn't mean the hard version is a lie, and just because the hard version exists doesn't mean every sniffle is the Black Death.
Which isn't to say that people are always smart about that distinction. Shit gets crazy overdiagnosed, because mental illness is scarier for the West, because we have this unstated assumption that everyone has the same mind -- that your thinking is exactly like my thinking. That we have different bodies is obvious to anyone with eyes, but that we have different minds -- different ways to understand the world -- is bloody hard for a culture that respects individuality and choice so fucking intensely. So if someone thinks different, it's scarier than if someone has a cough, it makes us question our own assumptions of equality (and homogeneity).
I mean, if that geeky kid isn't SICK somehow, what does it say about me that he's so good at computers and I'm not, right? If my child isn't ILL, what does it say about her that she can't get good grades like I did?
Over-medicalization of mental difference is a symptom of our fear of other peoples' minds. Which, again, isn't to say that this difference, when big enough, doesn't cause some actual problems, just that we're probably over-sensitive to the difference that is permissible, because WTF DO YOU MEAN YOU THINK DIFFERENT THAN ME WHAT THE HELL IS THAT?!?!