"[A kohen may defile himself] only for his relatives who are close to
him: his mother, his father, his son, his daughter, his brother, and his
unmarried sister..." (Vayikra 21:2-3)
A kohein may not go near dead bodies with the exception of those of his
closest relatives: his wife, siblings, parents, and children. It is a
mitzvah for a kohein to be involved in the burial of these relatives.
We learn from here that our relatives are people Hashem placed close to us
for our benefit - we should care for them, learn from them, and teach
them. We have to love them and be concerned for their welfare even more
than we are for others who are not as closely related to us (Rav Avigdor
Miller, Awake My Glory, p. 146).
The Sefer HaChinuch (mitzvah 264) teaches that mourning is a time for
teshuvah and self-improvement.