WWII did this in two major ways.
First, it left the United States as the major power in the "free world." The US was, generally speaking, opposed to countries having colonies. Therefore it put some amount of pressure on its allies to decolonize.
Second, it led to the Cold War. The Cold War put pressure on the West to decolonize as well. They wanted to look good in the eyes of the Third World. Having colonies would not help with that effort.
In these ways, WWII helped to lead to a situation in which decolonization could happen.
1-Due to Economic crisis in Europe
2-Poor living standard of the people due to poor economy
3-Poor Infrastructure in Europe caused by WWW II
4-Lack of raw materials in Europe because lot's of them were destroyed during the WW II