My response:Interesting question. I am the world's worst sick person. When I was a teenager, I had a hard-to-diagnose health problem that made me very sick for some time. During that time I lost every single friend I had.
When I am sick, all I can think of is me. Usually I have the energy to keep up appearances, keep the mask on, etc. When I am sick, I simply do not. I typically don't even notice my ill behavior myself. For instance, when I was sick as a teenager, I thought I was handling things remarkably well. I was honestly surprised when all of my friends abandoned me. Since then, nearly every time I am sick, I get into an unprovoked argument with someone. The sickness makes me less patient, more easily annoyed, which I mistakenly attribute to that person being particularly troublesome. Does this ever happen to you? Where you feel nauseous and equate it to being nervous, but really you have the flu? Or you are short tempered and equate it to other people being difficult, when really you have a migraine? This is what happens to me when I am sick.
I think my emotions are so low level that I have gotten used to reading changes in my body as signs of how I may "feel" about something. If my stomach is upset, I figure that I am probably nervous so my adrenaline is up. If I have a fuzzy brain or a headache, I assume that I am tired or overwhelmed. Because I have gotten used to doing this, when I am sick because of a virus and not just sick and tired of something or somebody, I mistakenly believe at first that I am responding to things going on around me. But they are just emotional hallucinations -- my body is tricking my brain into thinking that certain negative things exist, but they don't -- it's all just my brain misinterpreting data.
So I could see how your father might blame you or others for the discomfort he feels. Even if that isn't true, he certainly has much less energy to put on a happy face. What you are getting from him right now is the uninsulated, un-papered over version of him. And you're right that he is probably annoyed that life (and you in some weird way) has betrayed him, failed him by allowing him to become the shadow of the man that he was. But that must be pretty normal for old people with a touch of narcissism, I would imagine.

