I mean, I have no data for this, but I think it's a combination of two factors:

- Access to mental healthcare here is HEAVILY gate-kept by a combination of it often costing quite a lot of money, generally having longer waits than most healthcare services, heavy variability in availability by location, and insurance coverage runs the gamut between great or utterly absent, sometimes even in the same policy depending on what you need. Self-diagnosis is for many people the only diagnosis they have access to, and even if it's wrong, you can often use whatever diagnosis it is to find coping mechanisms that help you, or substances that will help you self medicate.

- Naming something medically is the only way to get institutions to pay attention to it, which can mean a number of things by itself, from getting appropriate accommodations at work or school, to getting certain kinds of coworkers or authority figures to treat you in a way that's more amenable to your mental state.

And I don't think its wrong to make it part of your identity either. Some definitely take it to a weird, unhealthy place, and also most of those people are teenagers. Teenagers do tons of stupid shit, I did tons of stupid shit. It's just part of growing up. But ultimately... it is part of you that you're going to be dealing with probably forever, so, some amount of identifying with it is probably healthy.

Edit: Also not sure how to read you calling this "oversharing?" Like I guess it could be depending what it is, but I dunno, my wife has BPD and a touch of Autism, she doesn't announce it when we meet someone but it isn't a secret either.

Everybody wants to be the one who overcame the odds or struggled bravely on. Everybody wants their story to be special because of how hard it was for them compared to everyone else.

I think some of it comes from the value the culture places on underdog stories and some of it comes from the oversaturation of everything in modern life.

Americans, and I am speaking in stereotypes and not about a particular person, don't respect personal boundaries on topics that other cultures would not discuss openly with someone they don't know well. Like for example, you tell me, a stranger on the internet, that your wife has borderline personality disorder. That would be oversharing if we had just met in a social setting and were discussing this topic together.