Helen Fisher: The Brain on Love  Ted Talks
http://www.youtube.com/watch?v=OYfoGTIG7pY
So many literary references about romantic love is what makes this extra fun for me.

I heard, "Want, motivation, reward lives in the reptilian center of your brain, and when you are denied something your brain in the wanting center becomes more active."  6min 29sec.

You're feeling intense focus, motivation, willing to risk it all.  She's talking about love, but could it also apply to other intense desires.