Hezekiah is the recipient of a personal miracle, but he also makes a major mistake. It must have been pride on his part to show off all his stuff. His response seems strange in that he is happy to hear that it is his descendants who will be defeated.
Elijah responded to a major victory with fear; Hezekiah responds with pride. How do we respond when God gives us victory? Ideally, it would be with humility and thankfulness.