So that's what it's called. I've felt that way for a good portion of my life, and only since I've started getting help in the past year did I realize something was actually wrong that could be fixed (it took someone else's genuine caring for a change to occur).
I originally thought I was just different from everyone else, or that "they" were wasting their lives on pointless pursuits. Now I actually understand why it's so great having a niece to take to the park.
It's like when a finger is numb -- you touch things with it thinking, "How odd, I ought to be feeling something." But you don't.
It's not just feeling down. It's your brain's ability to feel pleasure being physically broken.