why do humans forget?

why do humans forget?
tomorrow's homework
a promise with a friend
dinner you ate two nights ago
the lyrics to the song you supposedly love
the theories you memorized many times
the lesson plan you wrote many times
the name of the person you met just recently
the deadline of an assignment
the wide knowledge you learned from someone
fun memories
painful memories
memories of regret
embarrassing moments
sad memories
memories of crying so much your eyes got swollen
memories of nearly laughing yourself to death
memories of liking someone
memories of being loved by someone
memories of loving someone
memories of someone's death
is the part that controls my memory not working properly?
or is it just me?
growing older and considering every big insignificant thing as everything?
and throwing away every simple little thing as if they were nothing...
i start thinking things like, when we unexpectedly remember a memory of someone's death, is it because that person wants us to remember?
recently my cat's kittens were born, my sister-in-law got pregnant,and other lives were born...
one by one my memories are increasing
but i have to wonder
if these too will pass...
it's to God we'll always return.
amen.