I was once asked by a child, "Uncle, why do grownups look so unhappy all the time?" I answered the way an adult is supposed to answer: "They are unhappy because they have responsibilities, including having to answer impertinent questions from young people."
But I have since become ashamed of responding that way, and I have meditated on it. Part of the reason we look unhappy and tired so often surely is the slow realization of how things that have happened cannot be changed, that lost friends and family will not return, that missed chances at love can never return again, that deeds are done and words are said, even if forgiveness can mend many scars. A teenager, even a young adult, thinks that all things can be done, all mistakes can be repaired, a lost love will come through the door once more. But older people know that such things happen rarely, even with the best will, and that opportunities lost will not return, even though endless eons of time may unfold in the future. Why is the world this way? Why cannot these things be undone? Why does not the intensity of our wishing change the arrow of time? These questions mark the faces of the old. This is what we Buddhists of today think of, when we think of karma.
Pasanchon (contemporary Korean Buddhist -- trans.)