Ride solo for most of the reasons given here: have to fit it around family commitments, I am pretty slow compared to most groups I see on the roads; find the idea of joining a club a bit daunting. When I've done organised rides I have enjoyed the being with loads of others on the roads but, as I am on my own, have no obligation to try to keep up or wait for others.
That said. my wife got back in the saddle for the first time in ages at the weekend and it was good to ride with her. Yes, I was faster because she's out of practice but I didn't mind.