I love running alone as well. When I run with somebody, I get competitive and pick up the pace and get annoyed when they look better/don't tire out/go faster and then I end up beating myself up for sucking so much. At least when I run alone, I can focus on myself.

I hate cycling by myself. It sucks when you have a strong headwind, and it sucks when you are on a busy road alone. Plus, it is so much easier to just turn around and ride back home (or to the car) if you go out alone. At least for me.