I prefer the Machiavellian answer, because it seems to me to be more powerfully explanatory of the career arcs of Lenin and Stalin, the two principal protagonists of Soviet "socialism" in the last 100 years, especially in the light of their numerous and notorious policy zigzags which Alexander Solzhenitsyn, among others, documents in The Gulag Archipelago.
It's a little less fully explanatory in the case of the Maoists but seems to be effective enough for the activities and policies of the Vietnamese, Cambodian, and Indonesian (PKI) parties.
I prefer the Machiavellian answer, because it seems to me to be more powerfully explanatory of the career arcs of Lenin and Stalin, the two principal protagonists of Soviet "socialism" in the last 100 years, especially in the light of their numerous and notorious policy zigzags which Alexander Solzhenitsyn, among others, documents in The Gulag Archipelago.Socialists, Communists and other Statists always think that even though everyone has failed in the past, THEY are the ones talented and smart enough to make it work “This Time”.
I split the difference and go with the psychological explanation - they want power, and they rationalize to themselves as well as others that they are going to use it wisely "this time." Conservatives know that "this time" - when the socialists get it right - can never come.