It is entirely possible that a revolution in Russia would have occurred anyway; however Lenin formed the leadership and organizational skills to bring that revolution about. He was basically the right man at the right time. Had the Czarist or Provisional governments been stronger, it is doubtful that he would have succeeded.
By the time Lenin was sent back to St. Petersburg by the Germans in a sealed train in a deliberate attempt to undermine the provisional government, the Czarist government had already collapsed. A provisional government was established under Alexander Kerensky. Kerensky believed that continuation of Russia's participation in World War I was more important than reforms at home, a serious mistake. Soldiers were weary of the war, and peasants were ready to seize land from the landlords; in fact many soldiers abandoned the war effort to go home and join in the "land grab." Kerensky's government was further weakened by the activities of Workers Councils, or Soviets which were quite revolutionary.
By the time Lenin arrived, the country was in a state of complete anarchy. Lenin took advantage of the situation with his famous slogan of "Peace, Land, Bread," and attempted a takeover of the government which failed. He was then forced to go into hiding to avoid arrest. Shortly thereafter, another revolt was led by the army; Kerensky had no creditability with the army and was thus no longer in control. This gave Lenin a golden opportunity; his second, Leon Trotsky with a large number of Bolshevik followers seized government buildings, and declared Lenin the head of a new revolutionary government.
Lenin was a strong believer in revolution from above; only the strong could lead and should force revolution on the lower classes. In this instance, however, the revolution was already underway; Lenin simply provided the necessary leadership to bring it to a conclusion.
The revolution was already underway, Lenin simply hijacked it. What was more important was the aid given to him by the Germans during WWI and the offensive order by Kerensky, head of the provisional government, which failed completely. The failure caused the down fall of Kerenskys government and opened the door for Lenin, with the backing of Germany, to return to Russia and take over the revolution. Even then there were two groups vying for leadership, the Bolsheviks and the Soviets. It took several years of civil war to decide the matter. Of course we know who won :)