There were two reasons for the start of World War II.
First, and most importantly, there were three countries, two of whom were major powers, that were very dissatisfied with the status quo in the world after World War I. These countries were Germany, Italy, and Japan. Germany had been on the losing side in WWI and had been harshly punished by the Treaty of Versailles. It wanted to get back to what it saw as its rightful place as a major power. Italy and Japan had been at least nominally on the winning side in WWI, but they did not feel that they were being treated like true powers. They did not, for example, feel that they had empires that were large enough. When countries are unhappy with the international status quo, wars can come about.
Second, this was a time when war was still seen as an acceptable tool of foreign policy. Today, war between rich countries such as Germany and France is unthinkable. This is particularly true of wars whose main purpose is to take territory by force. In the 1930s, this was not yet the case. Our worldwide attitudes had not yet changed to the point where they are today.
Thus, the unhappiness of Germany, Italy, and Japan, coupled with attitudes that were more accepting of war, caused WWII to happen.