The dynamic range of emotions
"Dynamic range" describes the difference between the softest and loudest bits of a musical recording. If the sound was recorded poorly so that the soft and loud parts are similar, it stops being interesting. Imagine the 1812 Overture where the cannon fire was at the same volume as the brass, or Skrillex without the drop. Without softness to compare it to, you can't have loudness.
I was thinking about a loved one who passed away, and about the ebb and flow of happy memories mixed with tragic moments. The difficult parts were devastating, but I don't think I'd forget them if I could. Without the sadness to compare with, could the happiness be as wonderful? I wouldn't risk foregoing the lows if it meant the highs were less joyous.