I have two roommates who share a room, and they always go back there, close the door, and talk. Now, not that any of what they're talking about is any of my business, but a closed door is a pretty strong implication that whatever conversation is going on behind it doesn't concern anyone else on the other side.
So basically thinking about my roommates' closed door, I've come to realize that, metaphorically speaking at least, I'm on the other side of a lot of peoples' closed doors. There are so many people that I'd love to have discussions with and share ideas and secrets with, but they've all invited other people to their side of their closed doors. I have all of these acquaintances and friends with whom I only have superficial conversations. I've forgotten what it feels like to be genuinely close to someone.
I'm left alone, waiting for someone to open their door and invite me in.