I'm living in a flat with another guy. We are both in our mid 20s.
He recently invited a few people over. While I know, that I could always join, I didn't feel comfortable doing so without getting an explicit invitation. (I'm kinda afraid of being unwelcome, even though I know I wouldn't. That's why I need those explicit invitations.).
He then asked me, why I didn't join. I always feel uncomfortable to answer this, as I would've actually liked to join, but just didn't dare to. Me not joining might actually be kinda weird and unexpected from me.
What can I answer, that does not make it look like I'm disappointed he didn't directly invite me, but also shows I would've liked to join.
Right now I usually just don't answer at all or shrug it off which doesn't feel appropriate at all.
EDIT: One thing that I forgot to mention is, that our relationship is very good, I would even call him a friend, not just a flatmate.