I'm guessing it comes down to personalities, but that doesn't really answer many questions. I tend to be a responsive person in a conversation... I just listen until I see a fitting place to respond to something. I guess you could say I need to be prompted, but it's not like the other person does all the talking. I could dominate a conversation if I'm given something good to respond to. But I can't babble on my own. I think the trick is that you can't have two of the same type of people try to have a conversation. Between two responders, nothing will be said. Between two babblers, nobody will be able to finish a line. But if one person babbles and the other responds, it's a perfect match. The babbler can get the conversation started an the responder can jump in where appropriate. There are never any awkward moments of silence because the babbler can fill them in.
I wish I could babble when I need to.