For me very bad depression was just really weird.
All kinds of stuff went on for me (I've had PND twice) but I've just never understood that metaphor so makes me wonder how similar my experiences were to anyone else's.
I suppose what I'm saying is that this black dog thing seems to be the adopted way of describing depression and it sometimes makes me feel a bit confused that my experience of it was so different!
If that makes any sense at all....