We use the subjunctive when we want to express something contrary to reality. (Greek has a mood called the optative, which is a step farther back than the subjunctive; it expresses a wish. In English we would say something like “would that I were dead”—but I digress.) Okay, this being contrary to reality poses a problem for this guy, in a way, and therein lies the humor.
(This is Scary Gary (the vampire on the left) for May 2, 2017.)
Owen the ghost feels alive, so for him, being dead is contrary to reality, hence the subjunctive. The humor, of course, is in his misinterpretation of Gary’s question. But his grammar is perfect.
What I’d like to know is how Gary can balance a full coffee cup on the arm of an overstuffed sofa.