My goldfish refused to swim backward.

This joke swims straight into humor by using a classic comedic technique called anthropomorphism, where human characteristics are given to an animal. We laugh because the idea of a goldfish having a strong opinion about its swimming direction, let alone articulating a reason, is wonderfully absurd. The punchline relies on the fish's very human-like "logic" for not reversing course: it's already perfectly positioned for mealtime.

The real-world context here is our common understanding of goldfish as simple pets that mostly propel themselves forward. While some fish can indeed swim backward, it's not their primary mode of locomotion, nor do they typically ponder their orientation relative to their next flake. The humor comes from this delightful clash between a fish's actual instinctual behavior and the very human-like, almost lazy, justification for its actions. It's a silly, lighthearted take on pet ownership and the funny thoughts we sometimes attribute to our animal companions.