On each day of creation, God looked at his world and said that it is "good." On the sixth day, however, He looked at His world and said that it is "very good."
What, ask the Sages, was "very good" about the sixth day of creation? Their answer is that God created death, and death is very good.
Can it be that "death," in Jewish thinking, is the only creation that God refers to as very good?!
It's a strange concept at first glance, but when you think about it, it makes a lot of sense. We all know that if we have three months until an exam, we will do most of our studying in the month, week and days before the exam. Why bother three months before when there's still plenty of time? Only when the exam looms large do we suddenly feel the pressure to achieve.
Life is exactly the same. If you were going to live forever, why would you bother to get out of bed tomorrow? Why lose those extra 10 pounds now; lose them in ten thousand years or so! If life was eternal, we would have no motivation to do anything.
The fact that we have a deadline (literally) means that we know we need to get on with things. Of course, we could still ignore the deadline, often until it's too late, but nevertheless the deadline can motivate us if we allow it to do so. It's only because life is so short that we have the ability to use every moment of it.
Death is very good, because without it, life would not be worth living.