God left him to test him and to know everything that was in his heart

2 Chronicles 32:24-33

In those days Hezekiah became ill and was at the point of death. He prayed to the Lord, who answered him and gave him a miraculous sign. But Hezekiah’s heart was proud and he did not respond to the kindness shown him; therefore the Lord’s wrath was on him and on Judah and Jerusalem. Then Hezekiah repented of the pride of his heart, as did the people of Jerusalem; therefore the Lord’s wrath did not come on them during the days of Hezekiah.

 It was Hezekiah who blocked the upper outlet of the Gihon spring and channeled the water down to the west side of the City of David. He succeeded in everything he undertook. But when envoys were sent by the rulers of Babylon to ask him about the miraculous sign that had occurred in the land, God left him to test him and to know everything that was in his heart. 

Why?

People during peace and everything that goes well easily fall in pride, which may induce the wrath of the Lord.

Even during peace and great times under the Lord's blessings, God may have left us to test us and to know everything in our hearts. So some troubles and fails warn us that we depend on the Lord instead of on ourselves.

How?

Be humble at any time any period, even under difficulty my hope my heart, and my way rely on the Lord.