Our Gemara on Amud Beis discusses the principle that an Onan (a mourner who has not yet buried a relative and is in the intense early stages of grief and its preoccupations) cannot bring a shelamim sacrifice.

The Gemara’s proof text is a play on words from shelamim, which means whole or at peace:

“Rabbi Shimon says: The offering is called shelamim to teach that when a person is whole (shalem), i.e., in a state of contentment, he brings his offering, but he does not bring it when he is an acute mourner.”

Proper service of God takes place via emotions of peacefulness, joy, and contentment. For example, we have a tradition that Yaakov could not experience Ruach HaKodesh during the years he had unresolved grief for Yosef (see Rashi Bereishis 45:27 and Targum). Since God is obviously at peace and in a state of wholeness, we certainly cannot connect to Him if our minds and hearts are in bitter and dark places.

Yet, we sometimes must cry out to God from the depths of pain and suffering, as in Tehillim (91:19): “God, You will not despise a contrite and crushed heart.” Apparently, this is about breaking arrogance and facing what has been denied. This is a stage in a process, and still ultimately connection to God comes from wholeness.

Rabbenu Yonah (Shaarei Teshuva 4:8–10) explains the mitzvah of eating on Erev Yom Kippur in a similar way, balancing the gravity and solemnity of repentance with joy. Really, Yom Kippur is a Yom Tov and its mitzvos should be celebrated with a festive meal. However, this is a contradiction of terms, since the mitzvah is to fast. Yet, we ought to rejoice in both the mitzvah of repentance and in the recognition of what a great gift it is. Therefore, we celebrate it on Erev Yom Kippur.