First, Hitler (among many other Germans) assumed that they could take Russian territory without much of a fight. During the First World War, Germany had won one spectacular victory after another over the Russian Empire, bringing large swathes of Russia under German suzerainty. In many German nationalists' opinion, this territory had been wrongfully taken from Germany after the Versailles settlement and, given the ease with which it was first taken, could be quite easily reconquered. In a sense, the First World War had re-oriented German international thinking away from Western Europe toward the east, with an eye on Russia's large landholdings.

Second, Hitler was more than anything blinded by ideology and by his hatred of Russia. He considered the Slavic Bolsheviks racially and politically inferior and, consequently, no match for Germany. Furthermore, he was driven by his hatred of Slavs and Bolsheviks to make war on Russia as soon as possible. In truth, extirpating the Slavs and Bolshevism were probably Hitler's ultimate war objectives. Britain and France simply got in the war of this when they declared war on Germany.

Check out John Keegan's The Second World War if you want to find out more. It is probably the most authoritative and enjoyable single volume on the subject.

After the evacuation of Dunkirk there was little possibility of the British being able to open a second front in Europe (remember this is before America joined the war). Militarily and economically Britain was in no fit state to defeat Germany single-handedly in 1941.

In Mein Kampf, Hitler claimed that it is the duty of a nation to conquer more soil if its population needs it. Russia was the largest country in the world: "We must...coolly and objectively adopt the standpoint that it can certainly not be the intention of Heaven to give one people fifty times as much land and soil in this world as another". In order to ensure the German nation's survival, in Hitler's view, it would need to become a superpower like America, a contiguous land mass with hundreds of millions of people. The conquest of Russian land opened up this possibility for Germany.

Hitler's ultimate goal was to create a Greater Germanic Reich stretching from the coast of northern France to the Ural Mountains in Russia. The Slavic inhabitants of this coveted territory would be worked to death and exterminated to make way for German immigrants who would work the land as farmers. Hitler hoped this enormous state would grow to 250 million Germans in the 21st century.

I recommend Hitler's World View: A Blueprint for Power by Eberhard Jäckel.