Trump has been saying for a while that he wanted to meet with Kim. This is all part of Trump's plan as were the threats to cancel the meeting. It's just old fashioned negotiation tricks that Trump is very open about employing. The funny part is how well it works. An essential part of Trumps strategy is for whoever he is dealing with to think just what you do, that he is stupid and crazy and has no idea what he is doing and that he might just launch a preemptive nuclear strike. That gives him the upper hand in negotiations.