When I was younger, I used to be an extrovert: always trying to get a laugh out of the group, always speaking up, and giving lots of advice. In college I spent so many long, ardous hours studying alone, and needed to be by myself to study well. So I learned to be very happy and productive by myself. Since then, because of that, I am now more introverted and usually prefer to be alone, or with my husband. I recognize the value of having a few very close friends, as opposed to many "friends" who don't know me very well. So people used to give me energy, but now solitude does!
I also think it depends though, you know? It depends on what kind of situation I'm in, who are the people involved and how well I know them, and what we're doing or talking about.