Because the most common variant of Communism that took hold in the 20th century was Marxist-Leninism and their derivatives like Maoism and Guevarism. The core of Marxist-Leninism is the idea that a single disciplined party consisted of the most class-conscious individuals is required in order to bring the revolution to the masses and guide them towards the ultimate end goal of Communism. The party was suppose to relinquish power once society was deemed to proceed on its own. But in reality such conditions were never met for a variety of reasons and ruling parties proved to be reluctant to give up their power and privileges in any case.

At the same time Communists were determined to make mass, sweeping changes to society rapidly, which naturally provokes resistance. Ideology required that the state control nearly all economic activities within society and suppress private initiatives. Their revolutionary rhetoric and actions often provoked fear and the threat of armed intervention from outside powers because Communists sought to overthrow governments outside their own borders. The solution to all of the above tended to be a dictatorial government which at times have near absolute control over the countries they rule over.

Also bonus question, is it possible to engage in socialism without a dictator? thanks :)

This isn't really a historical question as it is a political one

But they don't - see many former socialist countries in Eastern Europe. Perhaps rephrase your question?