I don't know how one can even begin to describe the feeling. It is emotional, psychological, and a physical change in me when I hear that incredible song. (Which actually happens a lot [Ronny K does a lot of that for me]) But I think its simply incredible how sounds can alter a person so much, I think it is a beautiful thing.
The first song that got me into the dance scene (Used to be into Death Metal, strange change right?) was Vibraspheres Erosion. That is an INCREDIBLE song to me.
The song that got me into the Trance scene firmly was Walk With Me by Tritonal. That song, when I am sitting solidly more or less empty minded listening to music, still gives me shivers.
My top 5 (That I can think of) in no particular order
1: Daybreak-Tritonal
2: Galaxia 2009-Treadstone
3: Eternal Radiance-Tritonal
4: Walk With Me-Tritonal
5: Lost Together (AVB Mashup) Sophie Sugar vs Sunlounger
Most of these come into play in my top 5 because these were some of the first best Trance songs I heard once introduced to Uplifting/Emotional.
As for the feeling of a song going away, I have the self control to not ever listen to those rad songs that create such a change in me, so as to preserve that awesome feeling for years. I discovered Vibrasphere 2.5 years ago, and still Erosion will give me that same feeling. .. Electronic music is a wonderful gift. Long live Trance!