I searched the archive on this topic with no luck. Further, I am aware that there is probably no “answer” to this question, but I can imagine that there are leading theories.
Here’s the question, occasioned by the recent birth of my son- Why are babies fascinated by mirrors? It seemed that since about three weeks after birth, he was enraptured by mirrors, when very little else (including people’s faces) kept his attention for very long. Since so many baby toys feature mirrors, I deduce that other babies dig mirrors as well. Why?
I liked mirrors as a baby, too. My first words were “pretty baby.”
So my guess is, egotism.
(I have nothing factual to add.)
Just a guess, but it could be that mirrors are one of the very few things from which an infant can elicit a direct and immediate reaction. The image in the mirror moves in real time as the baby does. This might be the sort of concept that’s easy for the baby to connect; I move, image moves. Plus, a mirror comes with the bonus of mimicking human facial expressions (as much as the baby has learned them), which might be interesting to the child.
I suppose rattles fall into a similar category. The child can make something happen. With the mirror, there’s more variety, more consistency, more detail than with a rattle, or other toys.