Some lyrics are inexcusably stupid and a proud literate human being finds themselves incapable of making the mouth go through those insipid motions. Range is obviously an issue as a song that goes too high or too low both suffer without an individually appropriate key change. Sometimes a song is too excited or too depressed for any given evening. The songs might be too fast or too slow. The structure of the composition, the amount of effort put into rehearsal, all contribute to making a song right to sing.
Lord Malinov, Song of Songs