I can't count the number of times I've heard someone say "I'm just OCD like that" or "She's so bi-polar." And for the vast majority of people who say things like this, I assume they don't mean anything by it. Using mental illnesses as an adjective has become so completely common place that I don't think most people even bat an eye to it. However for a lot of people it isn't just an adjective. It is a chronic illness that we will live with for the rest of our lives.
When someone uses an illness as an adjective, they say that that illness is not in fact an illness but just a character trait or flaw. They invalidate the illness that millions of people face on a daily basis.
Don't get me wrong, I get how it can happen and I used to be a big culprit of this. We all have certain aspects of our personalities that could be described like a mental illness. Someone might be feeling a little depressed, or might like their things organized in a certain way. But what sets a diagnosed mental illness apart is that that is your default and your brain doesn't operate in a "normal" way without additional help.
Without the help of therapy and prescription medication my natural state is either semi-comatose on the couch completely detached from everything, or my body struggling to breathe because there are physically too many things to think about and a basic bodily function loses priority. To me that is not an adjective to be thrown around lightly. It is something that I have battled every day for more than 10 years, and that I will continue to battle until the day I die.
Our words matter. A cute little comment may not seem like a big deal, but it is. When I cannot leave a room until something is absolutely perfect because my anxiety is manifesting as OCD, I don't appreciate it when someone uses that condition to describe the way they like their clothes put away. It belittles the fact that I have spent countless hours of my life obsessing over perfecting one little detail (sometimes even something as small as my pen being parallel with my notepad).
We need to be more aware of the way we talk about mental illness, and this is just one of the many places we can start.