The popularity of eggbug has contributed tremendous evidence to my belief that humanity's platonic ideal of a cute thing is basically a fat oval shape with two dots for eyes and a simple mouth. You can create this magic object in like two minutes by poking at a sphere primitive and doodling on it and it could singlehandedly carry an entire subfandom in a game of your choice if it had hands, which it does not. It has roundness and a few facelike signifiers. If you put a thing like this in front of a human being they will die for it.
BB-8 is definitely related, but IMO it leans more on personality than design; it's not really cute until it displays cute behavior, even though its camera eyes approximate simple facial features. I think this specific phenomenon is something akin to
one of the good things about humans is that as long as we have enough clay for a round thing and a tool to poke a face in it, we can make a friend.