Generally, people are selfish.
They’re selfish in a quasi-altruistic fashion: they will do things for others, naturally, but they will also naturally favour their family over others’ families, their village over other villages, their region/racial or cultural affiliation over that. (Not denying the existence of genuinely wholly altruistic souls here, just saying they’re in the minority.)
In other words, it’s normal for an average member of the population to want to better themselves, their status, their material worth, and that of their family. Call this the Natural Profit Motive.
Communism attempts to upscale individuals’ quasi-altruistic behaviour to extend to the whole of its society or system. Consciously, or unconsciously, people simply aren’t motivated by this. If they work bloody hard at the steel mill or if they just do the bare minimum, they will get the same reward. So they tend just to do the bare minimum. (Qv. my experience in China in 1992, when the waiter asleep on the table told us to fuck off and find another restaurant for lunch - he’d get paid whether he served us or not.) It attempts to remove the Natural Profit Motive.
But generally, people still possess the Natural Profit Motive. Most individuals still want to better the situation for themselves or more likely for their family, but find they have no legitimate means so to do. Add a controlled economy that is unresponsive to market need, leading to shortages, and you have a situation that is even more likely to breed it: if I steal and hoard bath plugs now, when they become scarce I’ll become rich! Rich I tell you! Almost instantly they begin to indulge in black market practices (qv. chapters on shopping in Cuba, per The Cuba Diaries).
The second means to get on is to join the Party, lick the right asses, and ascend to the point where they get a more favourable education for their kids; a better apartment perhaps; maybe a limo.
So the authorities now have two serious societal problems to achieving their ends: demotivation, and corruption. How to counteract them without rewarding anyone financially?
How about trying to get people to emulate people who aren’t corrupt or demotivated. Step in the model worker: e.g. Alexei Stakhanov or Alija Sirotanovic.
So while Altruistic Joe Q Honest is ploughing away trying to become more like Alexei for the good of all, Asslicking Ivan Aparatchik has got himself into a position of authority.
New major problem: the best way to counter corruption is to punish people for corruption. But when party authorities are the people most likely to have corrupt and selfish motivations, and obviously the people who run the police will be in this position, how will this be positively enforced?
It won’t be.
So what other way can the Fearless Leaders fix this shit? Since they can’t make people feel good by rewarding them, and punishments are not meted out to the right people, why not make people feel bad if they don’t do something that the system wants them to do.
Love.
Love and devotion are also natural human emotions. And the majority of the world ploughs them into religion. But wait! Religion has been outlawed. Damn!
So what devotion can motivate the model worker, and turn the corrupt worker away from the black market?
Since the system is already perfect, then everyone must love the system. And if the system is perfect, then the leader must by implication be perfect.
Now we have an aswer: why do Aleksei and Alija work so hard? Because they are devoted to [insert leader here]. Why does Joe Q Honest not stockpile those lightbulbs? Because that would hurt [perfect leader here] and the system.
This policy grows. And grows. Pretty soon it’s scary to say anything about [perfect leader] at all; eventually people become so caught up in it that they worship said leader (qv. Chinese peasants burning incense, praying and laying flowers at the feet of the Mao statue in his mausoleum). Essentially they go completely mad, exhibiting behaviours that are pretty much unimaginable - outside a religious context.
And thus Communism becomes a religion, usually unsupernatural, but sometimes supernatural. Without the cult of personality, the whole thing would fall apart, because it’s again attempted to replace normal human behaviours with artificial ones, and that never works.
Mao, Pol Pot, Lenin, Kims Jong Il and Il Sung, Stalin, Castro, Ho Chi Minh (against his wishes) - few of them are immune. And when they do become immune to deification, that’s when the system crumbles - Brezhnev onwards to Gorbachev.