Sorrow and joy go together

Quote

“There are moments when I wish I could roll back the clock and take all the sadness away, but I have the feeling that if I did, the joy would be gone as well.” ~ Nicholas Sparks, A Walk to Remember

I find that memories of sadness are usually mixed with memories of joy. The joy that came out of an extended period of sorrow. Or a joke from a time that was otherwise painful. Or a tender moment amidst lots of untender moments. How to expunge one without expunging the other? How to erase one memory without erasing the others? Without the sorrows, would the joys still be joys?