I feel like a major contributor to the controversy over self-diagnosis is the half-awareness that is present in society. A lot of times you can see people behaving in a certain manner, and then comparing it to a mental disorder. For example, someone is not paying attention and behaving in a spastic manner. That person or people around them go, "you're so ADD dude." Or perhaps someone is organizing something or being neat, which prompts, "wow you're so OCD about everything."
The reality is, people are familiar with the popularized symptoms, but don't actually know much about the disorders. Some people are deciding that if they have some well-known symptoms, they have the disorder.
This detracts from the legitimacy of the disorders, especially when determined by self-diagnosis, because there are probably more people claiming to have the disorder than actually have it. It also causes issues because people then assume that everyone is making it up, which is very untrue.
An example of what I said in the first paragraph.
people have accused me of having OCD but i just like stuff to look nice and organized, it's not like i'm going to mentally collapse if it isn't