I can assure you, we don't - for some people this is not at all clear, and especially not without outside help.
I say all this because I deal with these issues to some extent too. But I don't give myself the out of "well I have depression". I'm responsible for my actions and I only have my one life. If something I do or refuse to do doesn't make sense and is making my life worse, I think about why and how I can stop it. I've gotten a lot of help and therapy over the years. It's not easy but the alternative is a lot worse.
Would you make the same argument about a partner with cancer, or Alzheimer's, or multiple sclerosis?
Beside that, there is some element of choice if the condition exists at the time you decide to make the other person your partner. If you meet someone with cancer and decide they'll be your partner, well, you make that choice and everything that comes with it. This is why I say I can't do it. I tried once, for a long time. If your partner becomes ill at some point after that, well, that's life. And it could just as easily have been you.
But anyway, yes, it's hard no matter what. Illness is bad, and sad, and it makes life hard for everyone.
You can tell when it's bad enough, like if there are visible tumors, but try to get 3 doctors to correctly diagnose whether a cancer is completely gone after treatment.
On the other hand, I guarantee 3 out of 3 doctors will diagnose my panic disorder as panic disorder.
I have recurrent tonsillitis, clearly a physical disease, yet 3 doctors couldn't figure out the cause.
When it comes to disease things are almost always murky, whether it's "physical" or "mental".
I was with someone like this for years in my 20s and it eventually became unbearable. At some level, "I have anxiety" starts to feel like an excuse, not that the person can control it exactly, but we all have the ability to tell which of the thoughts occur to us are good ones worth listening to and which aren't. I became resentful of how, for example, a nice vacation would be ruined over worrying about a lingering bill for $50 or whatever. It's really hard to be around someone like that long term and still feel like they make you happy and that your life is better with them in it, when it so clearly is materially not better. It's like all the color drains away from life and why bother doing fun stuff since it'll just get ruined anyway?