How are mental illnesses like medical illnesses?

No, I think you have misunderstood along the line. Even the mentally ill dislike pain. but some cause self-harm. It relieves tension. It's not that they like the feeling, it just makes them feel...

The question is rather pointless if you accept that the mental state is totally dependent on the physical state. Have you ever come across a great work produced by a dead writer? I thought not.

Diabetes is a physical conditionbecause it is the result of malfunctioning of the pancreas, an organ found in the abdomen that secretes insulin when high glucose levels are present in the blood,...

not really, but similar. they kind of go hand in hand

Yes