My sister and I sound JUST like you. We are now living together, but she would be happy on her own. I, on the other hand, wilt immediately when alone. Even when she leaves town for a few weeks on work, I become a sad, neurotic little plant.
Me, alone. No joke.
There are many things that contribute to my feeling this way, but basically I have a very hard time feeling safe anymore when I'm on my own. Even though my sister and I operate on opposite time tables and rarely see each other (maybe once a week?), it still comforts me to know I'm not completely alone. I'm incredibly blessed that she doesn't mind being around me, because I also can't live with roommates well (what--can't I have it both
ways??). I'm still a very independent, solitary person but just can't sleep in the house alone anymore.