For the same reason that most other countries hate the US. We're big, rich, powerful, & can impose our will on most other countries without much trouble. Many countries feel that the US has no business sticking its nose into their business. Sometimes they' have valid arguments but usually we jump in because of human right violations, governmental abuse or we're asked to, & of course the UN & NATO although constantly chastising America can't do anything without us. Kim won't do anything unless the Chinese give the okay (& they're not ready yet)
wow, all these answers are.. Just wow.. Its because of an old grudge, the cold war. America helped south Korea, which angered North Korea. Also, Korea is a propaganda only media (nearly) and the government needed someone to be the scapegoat for society to get behind hating. They chose western society, which generally equals America
Our involvement in the Korean War conflict 60 years ago, defending their enemy South Korea. Their hatred also helps rally their neighbors against us. By blaming the US for their problems, it justifies military threats & action that also harms their neighbors. It's all about power & control, perceived or real. NK has done this many times before to coerce economic aid from the US & our allies. It's no longer working, because of heavy economic sanctions, so NK's threats are becoming more serious. This sounds like another round of intensified blackmail tactics, for aid, & why nobody is kicking butt, yet. But everyone is still being watchful, just in case they're serious. (NK's actions may also be a distracting diversion. Gov'ts should also be paying attention to NK's allies. What they're up to right now. All eyes on this issue leaves opportunities for missing trouble elsewhere.)
Every country built on nationalism must be maintained using an "Us vs Them" mentality. Hate and contempt must be fostered and promoted in order to keep attention focused outward rather then internally. If all problems in a society can be blamed on a group, a nation, or a party, it is easier to hide your own shortcomings. N.Korea uses the "West" namely the US and S.Korea to achieve this goal just as the Nazi's used Jews, Christians used non-Christians during the crusades and inquisitions, etc. etc.
Because North Korea feels US is interfering in the internal matters of the country. US is trying to make war like situation by doing military exercises with South Korea. US is doing this because it wants to sell its weapons to South Korea and allies around it.