Why did Communism inevitably lead to dictatorship and totalitarianism?
It seems like Communist nations inevitably devolve into totalitarian dictatorships. Why does a left-wing pro-labor philosophy always turn out the same way?
Short answer: Because that's exactly what a communist government is intended to be.
Long answer (go fetch a cup of tea and some biscuits):
We must make a distinction between a proper "communist government" (indeed we should say "revolutionary government") and a government formed by a communist party. The latter is a democratic, elected government that will most likely abide by the Constitution and laws that are in place and will behave well, if not disturbed by reactionary agitation and "golpismo" (read my "huge, behemot answer" on What are the examples of democratic countries overthrowing their own democratically elected government by coups? to understand what this pesky word means). Such government, if powerful enough, will probably implement a lot of social, political and economical reforms (including some really and properly labelled "communist") but, since it will work under the constraints of a Constitution, it will most likely step down peacefully in the event of an electoral defeat. This type of "communist government" will not become a single party government or a dictatorship. There are plenty of examples of "communist parties" running democratic countries (usually in coalition) without much damage to democracy: Italy, Nepal, India, Brazil (our commies are currently part of the government coalition) and Greece are the ones that come to my mind. If you extend the label of "communist" to all parties left of the centre, then the examples are even more abundant: Spain (PSOE), Portugal (PS), Bolivia (MAS), Venezuela (PSUV), Greece (Syriza), France (PS), Germany (SDP and Die Grünen), Iceland, Canada ...
But if you mean that a "communist government" is one that is implemented by a revolution, then things are different.
You don't feel required to share power that was not shared with you. If you and a fried cooperate in a task, both of you feel entitled to share the pay. But if you have worked and your friend walked away, only coming back to claim his share, then you don't think it is fair to share your hard-earned bucks with him. That's more or less what happens in revolutions.
If the government shares power opportunities with every party and each has real and fair chances of governing the country and implementing his policies (that is, if such party gathers enough support) then no party will seek violent means to overthrow the government. Revolutions breed when a large and organised part of the population is denied access to power. You can avoid them by sharing power or by forcefully disbanding and clamping down on their organisations. Power sharing is more usual in Europe and European-style democracies. Disbanding popular organisations by force them is the usual in the USA (saludos, Occupy Wall Street).
However, to kick-start a revolution you need more than this. You need a government that is both corrupt, incompetent and cruel. Corrupt, so people lose respect for its institutions. Incompetent, to become unable to defend itself (weakness is a form of incompetence). Cruel, to anger enough people and convince them that only a violent reaction is effective. Review your history books and you will notice that revolutions happen against governments that are all of the three: the Somoza regime in Nicaragua, the Shah regime of Iran, the Spanish monarchy, the Porfirio Diaz government of Mexico, the Russian Revolution, the French Revolution, the Romanian Revolution, etc.
When such a situation comes by, there are three possible positions: to embrace the revolution, to fight against it, or to remain "neutral". Those who embrace the revolution will have to choose between forming a common front (unite) or lose the fight divided. This is the same choice of the ones that fight against. Whoever wins the revolutionary war will be unwilling to share power with the losers (these will be eliminated, if possible) or with the neutrals (these will be despised as cowards and opportunists).
Since the winners have reached all their military goals, they will feel entitled to implement all of their ideas. Why should they refrain from anything if they have won a war against all the rest? Fidel Castro fought a bloody six-year war against Batista's army. Batista was a cruel dictator (he once castrated a student leader and sent his penis and testicles to his fiancée) and shared no power with anyone. In Castro's view (which is actually reasonable), Batista's supporters were criminal, corrupt and unpatriotic (due to Batista's closeness with US interests). He then proceeded with the implementation of his measures, assuming that anyone who opposed them was actually defended the policies of his predecessor. You may think this is gross, but that's how revolutions work, and that's why every citizen of a democratic country should to his best to preserve democracy and prevent revolutions from happening.
An afterthought.
Even if this was not the case, a government that reaches power through a revolution under the banner of Communism will most likely pursue a Marxist-Leninist agenda, which is based on the "dictatorship of the proletariat" as a means to bypass the "bourgeois society" and achieve "communism".
Marx theorised that the "bourgeois state" was inherently adverse to the working class (he never knew any truly democratic states, as he died before the turn of the XX century). A "bourgeois" is a member of the merchant elite, the 1%, (the word is French and mentions the social origins of the elites of capitalist states, the mediaeval villages, bourgs, instead of the castles of the nobles). A bourgeois state is one that is controlled by the 1%, the owners of the "means of production", one in which the workers are either denied participation or can participate only limitedly. For instance, one in which working-class people are allowed to vote, but have no realistic chances of ever being elected.
Lenin took to extremes Marx's idea that the "bourgeois state" could only be defeated by an oppressive regime in which the interests of the working class where forced down on the bourgeois institutions. "Proletariat" means the working class, those who are totally, or almost totally devoid of property and political power. A "dictatorship of the proletariat" means a state that will do only what benefits the working class, either ignoring or directly damaging the interests and the position of the elite.
Communism is the goal of communist parties, it is a future society, after the dictatorship of the proletariat has finished dismantling the bourgeois state. In the communist society there will be no nation-states, no organisation above local community level (hence "communism", from the French "commune", which means "community", in the sense of place). To reach this state it is necessary to abolish all non-state institutions that exist across borders. Capitalism will be dismantled because corporations bypass state borders. If you dismantle the state without dismantling capitalism, then corporations will have more power than the people, because these will be divided across small communities. Corporations and capitalism have to be disbanded because their very existence prevents communism from being feasible.
To dismantle capitalism you have to seize the control of the "means of production" (land, real estate, machinery, factories, gold reserves, technology) and either pulverise it (land reform, for instance) or preserve it whole, under collective ownership (kolkhoz and sovkhoz as land structures in the USSR). After seizing the "means of production", the bourgeois state ceases to exist, but communism is still impossible, because production is still centralised. Communism is only possible, then, after factories, farms and everything else are evenly distributed, so that every region can provide for its own needs. While this is not achieved, the intermediate situation is called, diversely "Socialist State", "People's Republic" or something like that.
As you may have noticed, nothing of it can be agreed upon. That's why a revolution uses violent means to achieve its goals.
It is, of course, much better if the people can have their grievances heard and addressed by the government without the need of blood spilling. But some governments seem to disagree.