Reaching consensus requires finding a common ground.

In the case of a block size increase that common ground becomes the interests of Bitcoin (as a whole), but not the interests of its different parties, which often conflict with each other.

In order to agree on that one thing, we need to look at how Bitcoin started and how it gained value, what purposes different parameters of the system serve and how Bitcoin stands against the competition in the foreseeable future.

So, basically, consensus is not about differences of opinions and voting, but rather about finding a single underlying truth, sticking to it and understanding that benefiting Bitcoin (by agreeing) will actually benefit all of the parties involved.

Bitcoin shows us that being able to negotiate on complex issues and acting in goodwill is actually a mathematical necessity for progress and evolution.