Someone needs to be of sound mind to make a choice. Clearly you can't sign life-changing legal documents, or testify in court while drunk.

A range of other conditions, like deep depression, state of shock, etc. count. Does addiction?

Surely a definition of addiction is that you cannot stop, i.e. its a mental condition.

Like if someone ia diagnosed with paranoid schizophrenia, do we ask them if they want treatment? No, we have to treat then anyway.

I do not think this is a slippery slope, the conditions are fairly well defibed.