(TL;DR - self diagnosis for mental health is important and I'm sick and tired of seeing people be negative about it. Stop hating on self diagnosis because it's really important especially for young people and people of colour.)
Self-diagnosis is not a bad thing overall. It's actually a very important thing to know that somethings wrong with you and actively seek to figure out what's wrong with you.
Mental health is a really really tricky thing and unlike physical illness it's something that only the person affected by it can see a lot of the time. Depression, anxiety, eating disorders, borderline personality disorder, narcissistic personality disorder, etc etc are all things which require a self diagnosis before people can go and get help for them. (Or in the case of NPD a diagnosis from the people around the person with it who are affected and for them to take the decision to cut the person out of their life).
Also unfortunately actual diagnoses for mental health are hard to get. Especially in this country contrary to what some would believe. No medical professional would have diagnosed me with anxiety and they still don't want to and are making me jump through all kinds of hoops to get help even though I've literally been hospitalised as a result of it on a drip several times now.
I know several people who clearly have eating disorders and body dysmorphia - which btw are both mental illnesses that manifest themselves in physical symptoms not the other way round - who won't get the help they desperately need because their BMI isn't under 13. They won't even be acknowledged by 'professionals' as having an ED because their weight is not deathly low at which point it's too late for them to get help because the mental illness has such a strong strong grip on them and they relapse again and again. More people who get diagnosed with ED's die of them than people who are diagnosed of cancer.
Self diagnosis is so so important for so many reasons:
1) People need to acknowledge somethings wrong before they can go get help because mental health is not as easy as "Oh you have spots all over you you have chickenpox"
2) Some people can't go and get a professional diagnosis. And that can be because if in america they can't afford to. Or anywhere that they won't get the support from their family to do so. Dismissing a young person's mental health issues - usually POC tbh bc the older generation often doesn't believe in mental illness - because they haven't got a "real diagnosis" is so harmful?
3) Official diagnoses are hard to come by unless you're at rock bottom with mental health and are manifesting some severe physical symptoms as a result of them.
4) Self diagnosis allows you to go to the doctor tell them what you think is wrong and allow them to check and see if it fits.
Stop hating on self diagnosis because it's really frigging important especially for young people and poc.
I can accept that sometimes some people are like "oh yeah i have depression" because they felt sad one time. But A) that's not what depression is and they most likely are neurotypical. And B) most people who have a self diagnosis for mental health are either on the way to getting a professional diagnosis or are unable to and dismissing their mental illness because you don't think people can tell when something's wrong with their mental state is toxic thinking.