So in the vein of serious replies, I'll do the same.
Hitler primarily invaded Russia because the Ukraine areas of its western borders are amongst the most fertile in Europe. One of his main policies in regards to his own people was that of Lebensraum, or 'living space' that he felt the superior Aryan race was entitled to. What better place to claim as his own than somewhere his people could grow food and provide for their empire? Russia also represented a strategic threat on a scale that most other Western countries didn't pose at this point in the war. A pre-emptive strike, although we now know it to be folly, would have seemed a good idea because it would take one major player out of the war; also remember that Russia was Communist. Invading her most likely seemed a possible way to reduce the aggression of the Western nations who were strongly anti-Comm.
Now, the what if? What if Hitler had pushed westward? Operation Barbarossa began on 22 June 1941, at a point in time where Hitler had already crushed most of the opposition on his western borders, with the only real imminent threat being a beaten and isolated Britain.
Suppose for a moment Hitler consolidated his eastern flank in case of increased Russian aggression and turned his attentions and not inconsiderable forces towards the task that in the real world foiled him: the invasion of Britain. The invasion of Russia utilised over 4 million troops, 42 thousand guns and mortars (artillery pieces), just over 4 thousand tanks and a similar number of aircraft.
Dedicating this volume of offensive capability elsewhere would possibly have been devastating for the Allied forces on the West front. The Soviet Union, due to its vast size, was able to field its own military and airforce in numbers that gave it numerical superiority over the Germans and it's plausible that this is the only reason, apart from the terrible weather conditions, they were able to withstand the assault: throwing numbers into the grinder until the grinder stopped working.
Britain, by contrast, relied heavily on its airforce - one of the best in the world at this time - as its chief defensive weapon against invasion. Should Hitler have committed his previously USSR-focused air units to the Battle of Britain, he would have won. It's that simple. Skill and valour only go so far against numerical superiority, and under the cover of relay bombing runs there would have been little the British forces could do to prevent the landing of millions of German troops. Whilst I'm not saying it's a foregone conclusion, especially with US aid to the British, but it's very likely the British Isles would have fallen by 1943 if not before.
US involvement in the war would possibly have seen a turn-around in Allied fortunes but I personally believe that despite Japanese aggression in the Eastern theatre of warfare the American HighComm would not have gone through with an assault on the massive military machine of the Germans without some form of local support. A lot of the air successes met by US planes was due to their base of operations being in Britain - if this was denied, the chances of destroying the German economy and spirit are far reduced.
Draw your own conclusions from this as you will
