It could be depression... I could be fear.... (of the unknown - will I die? Why bother?) It could also be you've outgrown them... I find myself more social now, but I do find that some stuff I thought important before is not so to me now. So maybe they are somewhat frivolous? It could be you are angry or resentful towards them...? They are moving on with their lives - didn't have to suffer as you did. How can they have fun after all I've been through? Why me?

This disease changes your mind set.

Regardless detachment in some way is an insulator. If you are detached you cannot get hurt. Or maybe you've lost your joy (depression) get help - join a support group - a face to face one - or see a therapist to talk things out.

best of luck.