Changelings feed off of love because it can be produced by the changelings or others, and as such is rapidly renewable.  Without love, a changeling can and will eat most anything, even cats.

There are some changelings who prefer to eat meat, or maintain their old diets, on top of consuming love.  Others enjoy gnawing on substances that most changelings wouldn’t try to eat, like gemstones.  Right, ruuqo?