In a market-based society, your incentive to get out of your bed everyday to go to work is to make money out of your labor. You trade your time for money, which you get to keep, and then you can choose to trade that money for goods and services, or for future goods and services in the form of saving. You also know that the harder and smarter you work, the probability of earning a higher income increases as you become relatively more valuable.
Under socialism, nothing of this happens. You don't get to keep the fruits of your labor and you don't get to earn relatively more. The incentive to get out of the bed everyday to go to work fades away. This is where the dictatorship starts to play its role. You will go to work, because if you don't, the collective will force you to go, under a death threat.
As the markets are abolished, market-based prices disappear, causing the collapse of the economy. People start to wonder if there isn't a better alternative for their lives. They start to flee the country, but unfortunately the collective knows best. The collective needs you to stay and work for them, so they will shoot you if you try to leave.
Socialism always ends up a dictatorship because it's a human destroyer. Humans need to be pretty much enslaved in order for them to stay in the system.
