One of my favorite movies at Christmastime is Holiday Inn with Bing Crosby and Fred Astaire.  In the movie, Bing sings a song called "Be Careful, It's My Heart."  Aside from being a love song, there's an important lesson to learn about the fragility of our hearts.  When someone hurts you, it affects your heart.  Whether it be jealousy, anger, fear, or resentment, those things get lodged in our hearts and then affect our action against other people.  The truth is, when our hearts are sick and unhealthy, they cause us to treat others in ways that aren't honorable to God.  That's why warning comes in Zechariah 7:10, among other things, to avoid devising evil in our hearts.  Once it's there, it's hard to get out.  Forgiveness is so important for us because it heals our hearts.  Even if the person we're needing to forgive is unforgivable, it's better to say "I cancel your debt against me" than to allow all of the other emotions build up until you reach a cracking point.  So be careful – it's your heart!  Protect it, and empty it of any negative emotions each night before you go to sleep.  And if you've never heard the song before, you can check out the video from YouTube below.