"Ki holeich odom el beis olamo"
Because man is going to his day of death.
Koheles 12/5
Rabbeinu Yonah - EVERY DAY we are going on the path of our demise. Yesterday is DEAD. GONE. FOREVER.
Tomorrow - today will be gone forever. So let's make today ETERNAL.
How?
"Im bechokosai talaichu" - by going in the path of the Torah and toiling in Torah. We have the chance to made today last forever.
Or to waste it.
Gone with the wind....
It's in your hands:)!