Communism means group ownership
Communism is the abandonment of private ownership. All items and are held in joint ownership by the community and used according to need. Everyone works together to produce the grain and everyone then has bread for their sandwiches (To simplify matters 'grain' will be used to represent the various resources that people need to survive). This system has been observed to work very well in small groups (up to village sized), where everyone knows everyone else, there are high levels of trust and there aren't corrupting outside influences.
Group ownership requires trust
However, this doesn't work if some of the people aren't trustworthy. For instance, if you don't work as hard as your neighbour and only produce half as much grain then you don't actually lose anything - you still get your daily bread from central stores. Obviously, if everyone does this you end up with insufficient food in the stores and everyone would be on reduced rations, but each person believes that they can work less hard without any personal loss.
The problem is motivation
The question then arises - how can we make people more likely to do their share of the work? Well, people are motivated by one of two things - either they fear the repercussions of not being trustworthy or they care about what they are doing. The first case leads to the formation of tracking and measurement systems, as well as punishments and a centralised bureaucracy. This obviously detracts from the number of people actually producing grain (meaning everyone has to work harder) and nobody actually wants to live in a society ruled through fear by faceless bureaucrats. Also, in practice, most people don't actually believe they will be caught, so corruption would still exist.
People don't care enough
The second case is trickier - why don't people care enough about the village to work hard? Well, the answer is that most people just don't care that much about each other (see EmpathyCircles?
). If there's a common cause that people believe in then they will work together, During emergencies people will work together and make sacrifices for the greater good. However, as soon as the universal problem is dealt with, people's self-interests diverge again. When societies are more complex then you have the problem that not all jobs are equally needed or difficult - if you had the potential ability to be a writer or a doctor and enjoyed writing far more, even if doctor's were in short supply, would you be willing to go to medical school for 7 years and work 80 hour weeks in exchange for exactly the same rewards you'd get as a writer?
So, the problem is that people are lazy and uncaring, so we have to find another way to motivate them. And there's an easy one - people like to be rewarded for their work. If you link reward to work then people tend to work harder, because they're working for themselves. However, this means assigning more than the 'needs' of a person to them. If people have excess to their needs and don't have to share it with others, we're out of the realm of communism and into the realm of private property.
Property encourages people
When Vietnam legalised private ownership it went from importing 500,000 tons of rice a year to exporting 1,500,000 tons in less than 3 years - the amount of rice being produced went up vastly, because the farmers could see a reason to work hard. Similar things happened when China allowed villagers to own their fields and homes - suddenly they took an interest in their long term prospects and put the work in to make them liveable and stable.
Most people wouldn't work in their jobs unless they were paid to do them, and yet largely these jobs need to be done (if they didn't need to be done, people wouldn't be paid to do them). Until we can automate away everyone's jobs and produce all of our needs for nothing, we're going to need people to actually produce everything we want. Until that point, we're going to need to reward people for their work. Until that point we're going to need at least some capitalism.