The first time I rubbed one cat's belly, it looked at me like I was insane. "Why are you doing that?!" Thankfully, instead of shredding my hand into ribbons, it decided belly rubs are good things.

One particularly old cat always wanted to be petted but her arthritis was so bad you couldn't do anything but scratch behind her ears (or else it hurt). She had never been belly rub cat prior to developing back problems but she figured out pretty quickly that belly rubs are awesome because you get petted AND it doesn't hurt your back! Now she's all about the belly rubs. When she wants one, she will roll over on her back and make a sound I think that she thinks is "cute" but actually sounds kind of like she's being tortured. Or maybe more like a baby Alien.