Targum Yonasan: Are you very upset? Radak - Hashem did not say more. Rather, I will show you a sign that it is improper for you to be upset that I pardoned them. Heitev is the strength of a matter. The same applies to "va'Ekos Oso Tachon ha'Heitev" (Devarim 9:21), "Shiberu Heitev" (Melachim II, 11:18).

Radak: The good that I do to them - does it upset you?

Malbim: Why are you upset - because you think that they will think that you are a false Navi because your prophecy was not fulfilled? Did you prophesize about a good matter, that you should be upset if it was not fulfilled, for then they would think that you are false Navi, for such a Nevu'ah must come true, even if people sinned afterwards. Nevu'ah for bad, if they repent, the decree is annulled and the Navi is not shown to be false! This is clear from Sefer Yirmeyahu, from his words to Chananyah ben Azur (Perek 28).