what are north Korea's reasons to be mad at the U.S.?
The NK military elite trying to gain more support from the NK people for their 20 something year old supreme leader. Pretty heavy talk but I don't believe Kim Jong Un is pulling all the strings himself.
during the korean war we supported south korea
They are mad - period. Such kind of regimes need something to divert the people's attention from the obvious internal problems to a outside "enemy", who is responsible for all the misery.
Both of the above answers are correct. Jealous of the success of South Korea.