When everybody is sensitive to all the nuances going around, and not merely to what is happening in one's own mind, there forms a meaning which is shared. And in that way we can talk together coherently and think together.
Discussion really means to break things up.
It emphasizes the idea of analysis, where there may be many points of view. Discussion is like a ping-pong game, with people batting the ideas back and forth in order to win the game. In a dialogue there is no attempt to gain points, or to make your particular view prevail. It is more a common participation, in which people are not playing a game against each other but with each other. In a dialogue, everybody wins.