This is the way of the evil inclination. The biggest, most tempting fantasy you could think of--the minute it's over there's just no more desire. Nothing. And not only that, but it leaves a void in it's wake. The evil one laughs at us and when we are fool enough to give in to him. He leaves us with an emptiness and with no fulfillment whatsoever.
The opposite is true with spirituality. Before you do a Mitzva, learn Torah or say no to a non-Kosher desire, it can look hard, it can feel dry. "Pharaoh" comes to the person and says "Who is G-d that I should listen to him?" But afterwards, after you've given a little of yourself over to G-d, you are left with an inner fulfillment that shines and shines forever...