According to a recent Mainichi Shimbun report, Kim Jong Un apparently fires missiles on his capricious whims.
The Mainichi Shimbun is one of the main newspapers in Japan and they reported on the recent, brief sojourn of Kenji Fujimoto, who claimed to be Kim Jong Il's former personal sushi chef, into North Korea. During his two week stay in North Korea, Fujimoto had a little chat with Kim Jong Un, and Kim reportedly told Fujimoto that he "fired a missile simply because he was upset," and that he actually has no intention to start a war.
Kim also questioned his father's former chef what Japan's view on North Korea is, to which Fujimoto answered, "The worst." Kim reportedly replied, "Is that so," nodding his head as he continued to sip his red wine.