Why do childrens screams power the monster world?

This could be a very 'dark' question, or it could be whimsical.
The monster feeds on fear and pain
and thus the monster's power grows
Love is loss, and grief is gain
and pleasure comes when terror shows.

Just the reality of the 'Monster world'. a childs scream is a power source for their world.

Because monsters thrive on negative energy and frightened children's screams contain all that negative energy..

It turns out laughter is the best medicine after all.

Because you touch yourself at night

because it plays in nicely in the story