Beside this, why do I cry when singing?
You might cry while singing because there are certain memories or feelings attached to the song you are singing. Singing songs that evoke these feelings is an emotional experience and often leads to tears. It can also simply be that you become overwhelmed by the music and the song itself that you can't help but cry.
Likewise, can you sing while crying? When you're acting in a show, crying can be a powerful tool to help the audience feel your sincerity. When you sing, however, crying can make it hard for the audience to understand the words you're trying to communicate. For some performers, crying while singing can even be embarrassing.
Simply so, why can't I sing without crying?
You indicated that you don't think your crying while singing is emotional. If you actually cry as opposed to your eyes just watering and tears forming, then it is emotional. Tearing up of the eyes can be caused by the vibration of the front facial bones and sinuses caused by the resonating voice.
Why do eyes water when singing?
this happens because the song your singing can be emotional. Maybe you are so emotional and the song is so moving which causes you to tear. Once you sing, there is the nasal vibration which may indirectly cause the increase of tear excretion. That is maybe the main reason why your eye come out tears when singing.