I think it's because the snake seems so terribly keen on eating itself, in the same way that my rather young cat looks rather intense when he is imitating what he takes to be kissing, namely holding your fingers with his teeth, without biting.
The thing is, is the ouroboros fascinated by the biting or—and this is perfectly reasonable given where his/her gaze is placed—fascinated by being fascinated?
I'll add here that this is a major intervention in ouroboros imagination. Most ouroboroi are rather frightening looking, or rather opaque. The nearest one to Ian's is this:
It's not as good is it? I quite like the slightly defeatist ennui of this one:
But most others are terribly serious.