Early Enlightenment-era theorists of liberal democracy didn't have the same knowledge of psychology that we do. Rousseau thought that people were naturally good and reasonable when allowed to be free, and most calls for expanded democracy implicitly feature this as a basic assumption.

The reality, of course, is that people naturally self-segregate into like minded blocs, view outgroups with hate and suspicion, and are prone to groupthink and collective shifts towards extremism. Hence, deliberative democracy rarely works as advertised; what always ends up happening is factionalization and interminable conflict without progress.

Another problem I've noticed is that people often make the mistake of thinking that majoritarian voting democracy is inherently founded in peaceful relations and consensus-building, when the reality is exactly the opposite. The kind of democracy that our government is based in grew out of the implicit threat of military conflict between factions, and is analogous to ritualized agonistic behavior found in the animal kingdom. It is a system where people divide themselves into rival camps, rhetorically "attack" the "opposition", and then in the end perform a vote, which is effectively a display of force in which the losing faction is forced to back down and is often humiliated. If you look at the way we usually describe democracy and debate, it is replete with military metaphors; violence is always a subtext. That isn't the kind of procedure that breeds peaceful relationships and consensus.

If you look at institutions that really are centered around consensus building, like village councils or scientific communities, they do not have voting or democracy at all. They have small groups of people linked together by mutual respect and guided by norms that encourage persuasiveness and objectivity.