The 1978 version? It's self explanatory. He is still in love with his ex, even tho she is has moved on.
Oh honey! If you have never loved and lost you will NEVER be able to understand. Basically he's he's dieing because he KNOWS he has lost her forever.
I have always loved that song! It means he still has that feeling for her/ and he can't hide it when he looks in her eyes. Yes he has been able to move on but still thinks about her. That is my opinion.