There is one song that gets me to tears instantly. It used to be in my marathon playlist near the +/- 2.5h mark as a way to distract my thoughts from running, but it worked too good... most of the times I had to stop running and take a pauze because this song hits me like a truck.
To me, this is Sunrise from Norah Jones 
. I do like the song - but what hits me is the emotional memories it brings back to live. 3 years ago I was at my peronal ground zero - going into full (self)destruction mode and I eventually ended being admitted to different high care threatment programs. My GF (and kids) stood by me - regardless how difficult it was, they were there.
I didn't knew this song until we got married 2 years ago. My now wife picked this song to walk down the aisle. Saying yes during the bad times - hoping for the good times to come.
What is that one song for you? and what is your story behind it?