I agree for most mental illnesses and in most circumstances.

However I would raise two points:

1. For many people in America they may not be able to afford professional mental health diagnosis or treatment. Alternatively if living at home with unsupportive parents, they may not feel like they can ASK for mental health evaluation or treatment due to shame. This happened to me for my eating disorder.
2. There are some mental illnesses that you can self diagnose. For example, if I have obsessive thoughts about losing weight to the point it clouds my every waking minute for YEARS, feel self hatred about my body, restrict my eating, calculate the calories in every single bite of food, throw up into cups that I hide under my bed, only to later flush the contents down the toilet, all in an attempt to be thin, while beating myself up for even feeling this way because “I’m smart and should know better than to care about beauty standards” then I think I can safely say I have an eating disorder. Sadly when I was at my worst I felt so much shame about it, for being “weak” to fall for it, that I didn’t tell my parents. Later I did, and my mother mocked me, telling me that it was an affectation and to “get over it”. That’s why I didn’t get professional help. I wish I had sooner but I was a teenager. To this day the thoughts persist, though I’ve wrestled the actions under control. But if I say I had or have an eating disorder, I think it’s an accurate self diagnosis.

Not all mental illnesses are this easy to diagnose of course.