Why do good things come to an end?

It seems that we have been trained to expect all stories to end ‘happily ever after’.  I’m just not sure why.  I know why we WANT them to end happily ever after, even though we know it is not possible.  However things end, whether it is a beloved sport that you can no longer play, a relationship that is unable to be repaired, or it is death, the knowledge that it WILL end should inspire a different question.  How do I want to live?  What should I do with what is most precious to me?  Pastor Trudy