What was the main reason for Hitler to start World War II?

One of the main reasons was simply an expansion to the
borders of what he viewed as Germany, to take back the Rhineland and annexing Austria. 
After that it was to continue to further the goals of the Nazi party and the Nazi ideal,
to expand the borders of what Hitler considered to be a righteous republic while also
helping to rid the world of the people Hitler considered to be standing in the way of
progress and civilization.

Some might also argue that it
was simply a natural consequence of the poorly drawn up agreement ending WWI.  There is
also the argument that he had built up the military to such an extent that it simply had
to be used.