It's personal. Trump just bases his foreign policy on how much he likes various heads of state, and Kim knows how to manipulate a narcissist. Note how he adresses Trump as "Your Excellency" in that statement, as if he was a monarch? He does stuff like that all the time, validating Trump's feelings of grandiosity. That's how he's flipped him from the warmongering "Rocketman is on a suicide mission for himself!" rhethoric we heard at the start of his term to Trump believing he's done more for a peace treaty on the Korean peninsula than anybody before him.