I guess the reason why we treat mental illness differently is because it can severely impact ones judgement. Being unable to see things for as they truly are? That’s “crazy” and for a good reason. Pain medicine for my knee or allergy medication might have side effects — being more lethargic, for example — but taking certain medications that impact mental health, or not taking them, can massively impact the way one sees the world.
For example, I was taking an anti depressant, as well as weight loss medication. To make the weight loss medication more effective, I was given a supplement for it. This supplement plus my anti depressant? It made me fucking crazy. Like, scarily out of my mind. Everything at the time seemed perfectly logical and normal. But now I just have shame for the way I acted, felt, and thought for that period of time. Was it my fault? No. The doctor, when I told him about some problems I might be having, said something to the effect that when these combine it may cause emotional disturbance. Oh really? Good to know.
How do you explain that to someone without shame or guilt being part of it? Without worrying about how they will perceive every subsequent act you might make? It’s pure relationship poison.