It’s made me very sad, because I know it’s just the first of many times I’ll have to listen to him breaking that news.
Explanation: This joke plays on the nature of Alzheimer’s disease, which often causes memory loss. The humor comes from the sad irony that the speaker’s friend, due to Alzheimer’s, will likely forget that he already shared the news of his diagnosis, leading him to repeatedly break the same news. While Alzheimer’s is a serious condition, the joke uses dark humor to make light of the repetitive nature of memory loss.