service
Explanation
The Joke
A group of friends tell their friend Dave that they have all chipped in to get him a service dog to help him "deal with his problem." Dave, confused, asks "What? What problem?" The punchline is revealed in the final panel: the service dog is carrying a sign that reads "Dave, shut up about politics for like 10 seconds."
Dave's "problem" is not a disability or medical condition -- it is the fact that he will not stop talking about politics, which has become so unbearable to his friends that they have resorted to the dramatic measure of getting him a service animal. The fact that Dave does not even recognize this as a problem ("What problem?") is itself part of the joke, since people who will not stop talking about politics are famously oblivious to how tiresome they are being.
The Humor
The comedy works through the subversion of the "service dog" concept. Service dogs are trained to assist people with serious conditions like blindness, PTSD, or seizure disorders. Using one for the "condition" of being politically obnoxious deflates the seriousness of the setup and elevates a social annoyance to the level of a medical emergency requiring professional animal intervention. The dog holding a handwritten sign rather than performing any trained behavior adds to the absurdity -- this is clearly not a real solution, but an act of comic desperation by friends who have tried everything else. The joke resonates because nearly everyone knows a "Dave" in their life.