Why was Yisrael's reaction to the Voice of Hashem so praiseworthy?
Ramban: 'Because man has the choice to do good or bad, and the only thing over which he has control is Yir'as Shamayim'. 1
Ramban (citing B'rachos, 33b): 'ha'Kol bi'Yedei Shamayim Chutz mi'Yir'as Shamayim'. (As implied here, that Yir'as Shamayim is in the hands of the people, and Hashem was expressing the hope that Yisrael would retain the level of Yir'as Shamayim that they had reached then - Ramban: like a father wishes only what is good for his son - because then they would never sin.)