Why is the United States dealing with Kim Jong Un (Rocket Man)?

Why is the United States dealing with Kim Jong Un (Rocket Man)?

Russia and China both border North Korea so why is the United States dealing with this "Rocket Man" Kim Jong Un?

Shouldn't Russia or China deal with this Ruler that likes to shoot missiles over Japan?

And threaten to Shoot them at Guam?

Maybe the United States should cut off all the garbage that China imports to the United States.

And if the United States owes China trillions of dollars maybe the United States should tell China to deal with Rocket Man or we don't owe you anything.

I've never understood why the United States has to do all the dirty work for other countries?