In the second scenario, my ego tells me that I don't need the input of the other developer, because I know so much more what needs to be done. Both of these are the same problem, seen from opposite perspectives. The junior developer needs to realize that he'll never learn unless he asks questions and makes suggestions. The senior developer must remember that he needs to talk about what he's doing because it organizes his thoughts and he might be surprised with an insightful suggestion. Another common hindrance to conversant pairing is overdoing the controversy to the point that it becomes arguing.
Controversy is about searching out the best solution through taking up and defending sides.
Arguments are about ending up on top. At the end of a controversy, both sides have almost always compromised, and the solution is better than either side's original position. It is very important to detach oneself from the outcome and embrace it with a whole heart, whether it was your idea or not. When controversy begins to move towards argument, and either pair feels objectivity has been lost, or just doesn't know which is better, a third party should be pulled in from the team and their opinion garnered. This may result in one of the current options being adopted, or a third option may be suggested which is better than either of them.