If the things that once made you feel really awesome (sex, motorcycling, coding, gaming, cooking, whatever) no longer hold any appeal for you, that means you're very likely not merely "feeling down" but actually clinically depressed.
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.
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.