The answer to this question is based on the way that Chazal understand the system of Creation. HASHEM formed this world with definite and distinct laws: heat tends to rise, gases tend to expand, and heavy objects tend to fall. These laws are the bedrock foundation for all of physicality. Just as HASHEM created laws for the physical world, so too, He created laws for the spiritual world. These are specific and exact, and carry throughout Creation.
Before HASHEM created the world, He thought (if it could be) to create it with the middas ha’din – strict justice. However, the world couldn’t exist if it operated according to this system, so HASHEM created the world using the middas ha’rachamim – the system of mercy. The operating principle then became compassion. The way that actions were weighed and people were judged was now with a different scale and measuring rod. However, since HASHEM acts with complete honesty, justice cannot be ignored. It is mitigated and guided by kindness, but it still demands its due.
The Mesillos Yesharim explains that pure middas ha’din would demand instant punishment for a sin. After all, in this world, you are a creation, a visitor in the King’s land, created by and supported by Him. The King gave you laws for your good, and if you have the audacity to violate the express command of the King, even a slight transgression should be immediately punishable by death. The middah of mercy allows for a different way of judging an act: a sinner is given time to repent, the punishment isn’t as severe, and there is a system of tshuvah – of somehow undoing the sin itself.
However, din cannot be ignored, and there are times and situations where it comes into full force. For reasons that we humans will likely never understand, HASHEM runs this world in cycles and time settings. There are times of greater leniency, and times that demand more scrutiny in judgment. We are advised to daven on Yom Kippur with extra fervor because it is a time of greater rachamim. The same amount of regret and tshuvah on our part will accomplish more. The middah of rachamim is in greater force.
