I am of the opinion that going through a mental illness is at least equally difficult for the close family and friends as it is for the person experiencing the illness.
That might sound surprising, but there are a lot of illnesses that are like that, like dementia, for instance.
The reason I say this is because as a person who experiences extreme depression, months long bouts of insomnia, extreme agitation, and severe anxiety, I really don’t have much expectation of where I will be emotionally an hour from now, let alone tomorrow or next week. It isn’t in my nature to remain stable for very long.
Of course, a lot of the people closest to you deal with that uncertainty as well. And the defense mechanism for everyone around is to prepare for the worst. That is how humans deal with stuff. Prepare for the worst and you will at least always be prepared.