"Why are you celebrating the fact that you got diagnosed with [DISABILITY/DISORDER]?" I think you're missing the point here.
Being diagnosed means that you finally can see why you have always acted in this certain way, and not only could you change your habits, avoid certain thing that harm you, and take medication to improve your state in life, but also find a community that feels the way you have for years and accept you, and others, for who you and they are.
Being undiagnosed on the other hand is a BRUTAL experience. you are expected to do so much as a seemingly neurotypical person, that you know you cannot do, but are unsure of why. For all your life you were always alone or constantly bullied and yelled at since people see you as an outcast, and even you yourself are confused and ashamed about why you act this way, and could lead into much more serious actions like self harm or ever suicide.
Going to a psychiatrist and getting diagnosed gives you SO many benefits by simply knowing that you have a certain conditions. While it is hard to live with said conditions (I would know, I was diagnosed with Autism), it is harder, to live without knowing that you have it (Plus most of the issues we have with our conditions is just society and The System™ being ableist towards it.)