A natural urge that I can relate to, assuming you would like to finish by ejecting orgasmic spunk onto her. If not, and you'd just like to stand in front of her forever jerking off, then that's less understandable to me, but equally reasonable: she's very pretty.
Expressionless is good. Beauty, imagination, and perviness are all in the eye of the beholder.
I do like.Unreserve when ready!