Then to this earthen Bowl did I adjourn
My Lip the secret Well of Life to learn:
And Lip to Lip it murmur'd--'While you live,
Drink!--for once dead you never shall return.

Your hand can seize today, but not tomorrow; and thoughts of your tomorrow are nothing but desire. Don’t waste this breath, if your heart isn’t crazy, since "the rest of your life" won’t last forever.

Time will always exist but you will not. Stop your mind from drifting off into the future if you aren't there yet. Cherish the seconds of your life, live now not tomorrow. -Enlightened-

Well, it does make you realize that life, even at its longest, is still very short.

Why always "not yet"?  Do flowers in spring say "not yet"? 

Seize the day, take hold of it, and make it whatever you want.