The Yerushalmi (Makos 2:6) relates that Wisdom was asked what what the punishment of a sinner should be, and Wisdom replied that a sinner should be pursued by evil. Prophecy was asked what the punishment of a sinner should be, and Prophecy replied that the soul which sinned must die. Finally, G-d himself was asked the fate of a sinner, and G-d replied that the sinner need only repent to be absolved.
Is the idea of tshuvah a chiddush, a new insight, that is left to G-d alone to reveal? Are not our books of prophecy, Torah and Navi, filled with pesukim that implore us to do tshuvah, that tell us the power of tshuvah, that promise forgiveness for tshuvah? Do not our books of wisdom, countless gemaras, speak of the greatness of tshuvah?
I think the answer is that of course the power of tshuvah is no secret and is already revealed in all our seforim. But that power of tshuvah appears to be some sort of extra-judicial act of benevolance, a lifnim m'shuras ha'din that overrides true justice and causes the sinner to be spared. If you ask Prophecy, Wisdom, "common sense", what a sinner really deserves, undoubtedly the answer is, barring G-d's fantastic mercy, the sinner deserves to suffer for his wrongdoings.
However, G-d sees things from a different perspective, a perspective that we cannot understand no matter how prophetic or wise our insight may be. In our Yom Kippur tefilah we describe G-d as coming, "l'ta'her chosav ba'din," to purify in his judgment those who trust in him. The Sefas Emes explains: it is "ba'din," according to the letter of the law, that G-d purifies and forgives. It is not merely through G-d's extraordinary mercy that the sinner is forgiven, but to the contrary, it is G-d's attribute of justice that commands that the sinner who repents be granted a repreive.
It is this perspective on Divine justice that the Yerushalmi wished to share with us. The day after Yom Kippur is a Yom Tov, and seforim tell us that the simcha one feels on this day is evidence of the mechila granted on Yom Kippur. Only a deserved and just kapparah I think can give rise to a true sense of joy and happiness.