1)
Why did they rejoice?
1.
'Rashi': Suddenly, in Chizkiyah's first year of kingship, people returned to Hashem.
2.
Malbim: It was sudden, without arousal from below. They sensed that arousal from above prepared their hearts for Taharas ha'Kodesh and the many Korbanos.
2)
What is the meaning of "Al ha'Hechin ha'Elokim la'Am"?
1.
'Rashi': He prepared them to serve Him and do His will. This is like "Kol Levavo Hechin Lidrosh" (30:19). Radak - Al ha'Hechin is like Asher Hechin (that He prepared).
