If I get a cookie, then I’m happy because I got a cookie. The negation of this event is that I do not get a cookie. However, I am still happy because now I feel healthier, having not eaten a cookie today. So both the event and it’s negation cause me positive utility.
The negation of the event is that you did not get a cookie, not that you do not get a cookie. The negation of an event is that it did not happen. Either an event occurs or does not—it goes without saying that both an event and its negation cannot occur.
Even changing “do” to “did”, my counter example holds.
Event A: At 1pm I get a cookie and I’m happy. At 10pm, I reflect on my day and am happy for the cookie I ate.
Event (not) A: At 1pm I do not get a cookie. I am not sad, because I did not expect a cookie. At 10pm, I reflect on my day and I’m happy for having eaten so healthy the entire day.
In either case, I end up happy. Not getting a cookie doesn’t make me unhappy. Happiness is not a zero sum game.