Holy Scripture explicitly tells us to mourn the dead:
Sirach 38:16-22
"My child, let your tears fall for the dead,
and as one in great pain begin the lament.
Lay out the body with due ceremony,
and do not neglect the burial.
Let your weeping be bitter and your wailing fervent;
make your mourning worthy of the departed,
for one day, or two, to avoid criticism;
then be comforted for your grief.
For grief may result in death,
and a sorrowful heart saps one’s strength.
When a person is taken away, sorrow is over;
but the life of the poor weighs down the heart.
Do not give your heart to grief;
drive it away, and remember your own end.
Do not forget, there is no coming back;
you do the dead no good, and you injure yourself.
Remember his fate, for yours is like it;
yesterday it was his, and today it is yours.
When the dead is at rest, let his remembrance rest too,
and be comforted for him when his spirit has departed."
Sirach 22:11-12
"Weep over the dead, for their light has gone out; weep over the fool, for sense has left him.
Weep but less bitterly over the dead, for they are at rest;
Worse than death is the life of a fool.
Mourning for the dead, seven days —
But for the wicked fool, a whole lifetime."