Good boy
Explanation
The Joke
Two dogs are talking in a forest. One dog argues passionately: "But if all boys are 'such a good boy' then the term is without meaning!" The dog is applying logical reasoning to the universal praise that dog owners give their pets. If every dog is told it is a "good boy," then the designation loses its semantic value -- it fails to distinguish good dogs from bad dogs, making it a meaningless compliment.
The comic imagines dogs as being philosophically troubled by the indiscriminate praise they receive from humans. Rather than simply enjoying being called a good boy, this dog has thought deeply enough to realize that universal application of a superlative renders it vacuous. The other dog appears to be listening while holding a bone, seemingly unbothered by this existential crisis.
The Humor
The humor comes from applying rigorous philosophical reasoning to the most mundane aspect of dog ownership. The dog is essentially making a valid logical argument -- a distinction that applies to everything is a distinction that distinguishes nothing -- but about the least consequential possible topic. It is a parody of philosophical discourse, placing Syndrome's logic from The Incredibles ("If everyone is super, no one will be") into the mouth of a distressed dog. There is also something inherently funny about imagining dogs having intellectual anxiety about the compliments they receive, when in reality dogs are famously and endlessly delighted by exactly this kind of indiscriminate praise.
References
The logical principle the dog invokes is related to the concept of "distinction without a difference" and echoes ideas in philosophy of language about how meaning is derived from contrast. If a word applies to all members of a category equally, it provides no information and is semantically empty. This is related to the Fregean idea that a concept with no boundary has no content.