There's something compelling about reflections and mirrors. Narcissus stared at his own reflection in a pool of water until he died, while Alice used the mirror to travel to Wonderland and meet fantastical beings. There seems to be a promise of something more interesting just on the other side.
Unless you're a kitten. Then the reflection is just some weirdo cat that keeps disappearing when you walk away from the mirror.
As this video shows, the kittens respond to their reflections much like Narcissus in that they think their reflection is another cat, just one that's mimicking them. While a few are just curious and paw gently at the glass, most of the kittens arch their backs and fluff up their tiny tails to scare off the "other" cat. While a cat may eventually learn it's just a reflection in the mirror, the tumbling, stalking and chasing of reflections by these kittens makes for a sure-fire pick-me-up for us humans.