I've always had roommates. Sure, I can get annoyed with the constant presence of someone else in my own private space as much as anyone, but having roommates has always forced me to be a more social creature than I am. When someone's awake and in the living room when you come home from work, it's just plain wrong to avoid at least a small amount of conversation. I've also been shaped by the fact that I'm a middle kid, and I've never known a place where I wasn't surrounded by other people who are looking for their share. I'm just not used to occupying any space that isn't one small room.
Some people can breeze into any living situation and make their presence felt.
I've lived with folks who have taken over the minute they waltzed into an apartment. Changed the furniture arrangement, made room for their food in the kitchen, adjusted the thermostat and invited thirty friends over for a party every other weekend. Me, I'm more the type to try and blend in. Everything I own could still fit into a single room, albeit a rather crowded one. I'm slowly starting to realize that this place is all mine, and that I can stretch out, if I want to.