I have ME/CFS rather than fibro, but I think any chronic condition can cause you to lead a pretty lonely exsistence. I've had a lot of bad reactions from people in the past regarding my illness which puts me off getting too close to most people. The fact you can't physically leave the house so often certainly has an effect, but one of the biggest things (for me at least) is that when you feel like rubbish and are completely exhausted, you lose interest in the outside world and lack the energy to really listen to people/make conversation etc. I don't mean that to sound mean or anything, I don't stop caring about people, I just literally do not have the energy.
Most people wouldn't want to go out with their friends when they have the flu, and you feel the same with a chronic illness, the problem is that it doesn't go away! To a certain extent you try and learn to adapt, but there's only so much you can do. I hope your friend does feel well enough to get out soon, and you are a lovely friend for going to visit him so often. I have lost the vast majority of my friends through being ill, they soon forget about you once you don't fit into their world anymore, and I'm sure he really appreciates seeing you.