The spread of class consciousness and the organization of the masses.

No matter how bad the circumstances get, they won't automatically and systematically arise. There will be plenty of spontaneous revolts, but as long as they are not gathered under the same flag and centrally coordinated, they will be crushed by the ruling class using the whole might of the state. It needs a vanguard party that can oppose the state of the ruling class by uniting workers' struggles all across the country. This can only be a result of educating the masses by arming them with the right communist theory and building a fighting organization that can translate that into communist praxis, the communist party.

Or the short version: no matter how bad the objective factors get (cost of living, police brutality, endless wars), it needs the subjective factor to do the revolution (the organized proletariat under the flag of communism).

At least that's what Lenin says