Keep fluffing and I'll poke your eye out with one of these sharpened forks.

I think it depends on the degenerative state of their mind.
To begin with, something like 1 in 3 people suffer from a mental illness: depression often accounts for this, but it can be anything from Prader-Willi syndrome to dementia, making it very difficult to lump everyone with a mental illness into one group.

Peopl with depression rarely love themselves, but they can love others. However this love is not often good for either person and detrimental to the relationship, or it can be an obsession disguised as love.
I don't have experience with all depressed people on the planet but from what I have seen relationships in which one or both parties are depressed are rarely functional, and most don't last without some bind of marriage or children or fear (mental/physical abuse), and even that doesn't always hold it together.
There are though, always exceptions to any rule, and I'll probably be disagreed with on this point.

I have to go to work so I'll see if I can do some more later.