What is the theme of "Lament" by Edna St. Vincent Millay?

"Lament," by Edna St. Vincent Millay, seems to be the
address of a mother to her children, discussing the recent death of their
father.

The mother gives two seemingly contradictory pieces
of advice to the children about how to cope with the death of their
father.

The first piece of advice is to hold on to little
items that will help them remember their
father:

From his old coats
I'll make you
little jackets;
I'll make you little trousers
From his old
pants.
There'll be in his pockets
Things he used to put
there,
Keys and pennies
Covered with tobacco;
Dan shall
have the pennies
To save in his bank;
Anne shall have the
keys
To make a pretty noise
with.

On the other hand, the
mother advises the children that "Life must go on, / And the dead must be
forgotten."

Continuing this vein of advice, the mother
urges the children to continue with the mundane details of
life:

Anne, eat your breakfast;
Dan,
take your medicine...

The poem ends
with a statement indicates the mystery of life and
death:

Life must go on;
I forget just
why.