I enjoy listening to music, and, as an amateur musician, playing and writing music. In my lifetime, there's no telling how many songs I've listened to, or how many times I've listened to them. It can sometimes be difficult for a songwriter to write a song that doesn't sound too much like another, already existing song. But there's another problem, even for the casual listener, in just remembering how a song or tune goes, even if you're just thinking about it or humming it.
A really distinctive tune or melody can be so powerful that it obliterates any other tune or melody that you're trying to play or remember. Amazing Grace is one such song. It has such a strong, simple melody that anything that sounds anything like it can just be obliterated out of your mind and taken over by its melody, instead. You can start humming the melody of another song, and it quickly turns into Amazing Grace. And then you can't get Amazing Grace out of your head.
I call this Amazing Grace Syndrome. Heh.