The traits and habits I tend to notice in people who make and keep friends easily are usually (in my mind) negative: willingness to talk at length about inane, uninteresting things, lack of independence in pursuing hobbies and interests, the ability to BS easily, fondness for or even dependence on alcohol, etc.
I'm sure this makes me sound like a snob (I've heard that before), but I'm being as genuine as I know how to be. I really don't understand how other people have the patience for the kind of crap that has to be put up with to maintain a large circle of friends.