My weekday commuting is always solo. On weekends, it depends whether my wife decides to ride with me. If she wants to ride, that's great, if not, I'll go solo. Her pace is slower than mine so I enjoy solo rides as I can push myself more. I will try to pace other riders that I happen upon (always from a safe distance - 50'+ so it doesn't appear I'm drafting). Somethimes I've got no problem keeping up, sometimes they drop me pretty quick.
I wouldn't mind riding with a group but I enjoy leaving when I'm ready and starting/finishing rides from my garage. Riding with a group would require scheduling and I really enjoy going where I want, as far as I want, as fast (or slow) as I want, when I want.