user
Given a cartoon image and a caption. Caption: Last time I'll book a discount vasectomy, for sure. Question: Generating an explanation of why this image/caption combination is funny. Answer: 
assistant
Spaying/neutering pets is a commonly performed operation for animal sterilization performed at animal shelters; the human equivalent sterilization procedure is called a vasectomy and is usually more expensive due to higher quality, human medical requirements. But here, the two men are trying to save money by having their operations done at an animal shelter. It's funny in part not only because this is an unusual way to save money, but also, because vasectomies only are performed once per person, so this is literally the last time the person will do this procedure, even if they liked it; the quote implies they aren't enjoying being locked in an animal cage.