I tend to play alone because i find it faster. Im kind of Selfish when i game because i want to do things i want to do, i dont really want to have to do what group members want. for example, I would rather go in a straight line than go in a wavy line(if you can picture it in your head). If group members just followed me and helped me kill things than it might be a different story. but they always end up not having this heart or that skill point or whatever.
Btw im not really like this in RL, just something about gaming makes me kind of selfish in that aspect.