There are different kinds of work which needs to be done for our society to function. These tasks have costs for those who perform them (lost time, spent energy, danger, boredom, etc).

In pure communism, everyone works hard and everyone is given the spoils of the work we collectively provide. But it is rational for any individual to not work as hard, because he will bear less of the cost of that work, but still realize the same gain

Therefore most people tend to shirk their duties, and the output of the entire collective drops. In order to maintain the system, the threat of violence is introduced, and we quickly get to Stalinist purges