One thing to keep in mind is that people were really not happy with the Czar/absolute monarchy in Russia in the runup to the Revolution. There had been revolts in the decades leading up to the February Revolution (thinking of 1905 specifically), and the war wasn't going well.

It's also worth remembering that Russia did not go Bolshevik in one fell swoop. First there was the "February Revolution", when the Czarist system was overthrown and a democratic provisional government was established. It wasn't until months later that the Bolsheviks came to power within said provisional government and Russia officially became "Communist" as opposed to some other political alignment.

TL;DR: It's not like the whole country woke up one day and said "The Czar sucks, let's be Bolsheviks!" A lot of specific events had to happen to bring about the USSR.

On the ideological side, it was, as The Geoninja mentioned, always a matter of building - first socialism (USSR proclaimed socialism as achieved in its 1936 constitution), than communism. But communist end of history was always in some undefined future, always for the next generation.

That is a condensed view, so keep in mind that ideological ideas changed significantly on that matter. After October, a world or at least European revolution was expected. When it didn't happen, Stalins' socialism in one state followed. World revolution was put somewhere in the future, as undefined as communism itself, and USSR was only to wait for it, provide an example and a leading light (a City upon a Hill image of America is quite appropriate here as well). However, apart from some minor opportunistic help to revolutionary movements, that were geopolitically rather than ideologically motivated, it was not to make the world revolution, only to wait for it. Making it happen was more of a Trocky thing. After the Second World War there was a significant movement towards socialism (with potential future communism) in agrarian and Third world countries. While significant for idealist expectations of a world revolution (amongst Western European "Maoists" for a well known example), for actual revolutionary countries it was more of an attempt to achieve rapid industrialisation and modernisation on the 1920s and '30s Soviet model.

It can also be useful to thing about "Marxism" as a practised ideology as a faith (Eisenstadt, "Fundamentalism, Sectarianism, and Revolution" merges both socialism-as-a-modernisation and socialism-as-faith interpretations quite well).

Another important thing to consider it that, just because really existing socialist systems did fail, and terribly at that, they were not always already failing. Early successes were a major factor in what kept them going despite inability to go from extensive development (that they did extremely well) to intensive development (that seemed unobtainable in socialism).

Initial Bolshevik program (in 1918/18) was summed up in a parole "Peace, land and bread" and they managed to provide that, mostly. Rapid industrial development followed, and that was repeated in most of the later socialist revolutions, both imposed on East Europe by USSR and autochton ones such as China. (I like Lane, "The Rise and Fall of State Socialism" because it shows both the successes and failures of state socialism very well).

So, in a sort of TL;DR, faith in the future and successes of the past both had a major role in keeping state socialism, and hopes of future communism, going. Additionally, life was not simply a grey dystopian drudgery. On that point, I can highly recommend Yurchak, "Everything was forever, until it was no more". It is a long but very interesting read, basically showing how in later decades in Soviet Union it was a case of the end seeming unimaginable, while at the same time creating a situation where collapse once it did occur surprised almost no one. Also, it is an excellent work on Soviet everyday life of normal persons.

Just an important note to add. The Soviet Union really didn't achieve communism. Just a form of socialism that some might state differed between leaders.