Are there reasons why an invasion of Switzerland didn't occur? In looking at Axis Occupied territory maps, it seems almost odd to have a sole country in the middle of Europe still exist. I would think the same thing of Spain, but from what I understand Franco leaned towards the Axis powers, which probably kept them at bay from invading his country.
Germany and Switzerland before and during WW2 had a tumultuous relationship. Hitler hated, and I mean hated Switzerland. He commonly referred to them as "pimple on the face of Europe". The majority of the people of Switzerland fell within Hitler and the Nazi's definition of an "Aryan" but in Hitler's view, they had been corrupted by years of democracy.
After the fall of France and the securing of the western flank the OKH (the German high command) ordered plans to be made for the invasion of Switzerland. The invasion was to be carried out by Army Group C commanded by General Wilhelm Ritter Von Leeb. He began to survey the territory and plan an invasion route, the plan was to quickly crush Swiss resistance and partition the country with Italy, who desired the Italian speaking parts of Switzerland.
However the plan was postponed indefinitely. The main reason was that it was simply considered unimportant in the grand scheme of things. Hitler assumed that once the Soviet Union and Britain were dealt with, he could than turn and finish of Switzerland. There were other reasons of course. Switzerland was obviously not an easy country to invade nor to rule. Switzerland also had control of the Gotthard and Simplon tunnels which were necessary for the Axis who used them for rapid transportation from Germany into Italy and other areas.
Finally, Switzerland also provided Germany with economic incentives not to invade, but these were arguably of a less important influence on the decision not to invade. Germany could trade with Switzerland and get goods through Switzerland that it couldn't normally get because of the allies economic blockade of Germany. Swiss industry (which was received lots of supplies from Germany, namely coal) also helped supply Germany with weapons and other armaments; war profiteering in Switzerland was common. As German industry came under increasing pressure from allied bombing, Switzerland was considered necessary to keep Germany's war machine going. Eventually the allies caught on and threatened Switzerland into stopping arms production for Germany in October of 1944.
A good source on the matter of the neutrals during WW2 is A world At Arms by Gerhard Weinberg