Discussion, because it is an activity, engages the student's mind more than do lectures. Instead of passively hearing the professor's thoughts, the student engaged in dialogue is required to work his own mind, to form and express his own thoughts. And the challenge of discussion pushes him to ever higher efforts. He reads more carefully knowing he will soon be explaining and debating his own understanding of the text. If he does this on a daily basis, this heightened intensity leads to intellectual habits that last a lifetime.
Lectures are less likely to produce the intellectual virtues.
At best one might remember certain ideas from a lecture, but they have probably not become part of him. But the ideas he has worked out himself are likely to stay. And not only to stay but to become part of his mental makeup. Or again, the flow of concepts presented in a lecture is often too much to assimilate. Sensing this, a good lecturer will repeat key points and connections and even invite interruptions when the audience has lost the drift.