Well, when we study a piece, we must achieve the technical control of that piece, and so we read the sheet music, we look for the most dificult bars, we repeat some bars a lot of times... i.e. we transform a wonderful piece of music in a "case study" and this along many days or weeks. It`s very natural to loose the initial fascination...
There`s another problem: in all we do or wish, when we have it always we wish another thing. As human beings, we have thirst of infinite...
So, I use to do like this: after "learning" one piece and when I can play it very well, I let stand it a while (one week, one month...) and I only return to that piece when I mey "feel" it again.
But when we play in public we are allways more "mind" than "heart", I think...