One time, I performed a song for a small group of people, and it was a very personal song, so I was nervous to sing it well. As I took the stage, the tears welled, and by the second verse I couldn't hold them back any longer. My voice wobbled and cracked, and I felt like the entire room suddenly had all the air sucked out of it. It was mortifying.
Music is emotion in one of its purest form. But there is an important balance between expressing the emotion of a song effectively, and having so much emotion that it becomes difficult to sing the song to its full potential.