Ever since I was a small child, I have sporadically experienced this pleasurable tingling sensation that starts in my head and gradually works its way down my spine.  It varies in length and intensity, with the strongest sensation being in the head and face area.  In my case it would usually be induced by watching someone do a simple or benevolent action, and/or articulate innocent commentary.  A fairly intense one in my memory is when I was cuddling with my girlfriend at the time, and she started cutely explaining how she had applied nail polish earlier that day.  There was something extremely wholesome and attractive about how she spoke that induced the sensation.  It's a feeling of well-being unlike any other.