Desire.
In velvet nights he twists within his sheets. Can the Potions Master get it up? The conjectures of his students do not touch him. Lord Voldemort's experiments with eternity and soul took something from Severus long ago. What was taken could easily have been put back and hence Riddle's conviction that Snape would not return.
Not so. A bottle of deepest blue reposes on Dumbledore's shelf.
All these years.
Intelligence and anger could never make good what's missing and still the absence aches. Severus thinks it would be better if he had it back but doesn't desire it.
He cannot.