While I'm not in favor of self diagnosis because its negatives outweighs its advantages, I do understand people who do it. Like Clay said, it's often not a choice. Many people can't afford a professional diagnosis and others are scared of the stigma.

In fact, stigma against disability, chronic diseases or mental health issues become a hurdle a lot. It can take the form of judging a person with a disability based on misconceptions, assumptions, or stereotypes and concluding that they are less valuable or capable than an individual without a disability. This can impact one's career by causing them to not apply to dream jobs because they are unsure if they can work long hours under pressure, for example, or by leading them to overwork at the expense of their health. This stigma can have a range of negative effects, including internalization of negative beliefs, social isolation, low self-esteem, shame and avoiding treatment.

It can also lead to further marginalization and reduced hope, as well as difficulty finding housing or work opportunities. Stigma can also contribute to worsening symptoms and reduced likelihood of getting treatment.

In fact, most countries don't accept immigrants that are disabled or have/had mental health issues. Now with this kind of marginalization, why would someone want to have their illness on paper?