I visited the doctor because I couldn't sleep because I felt like I was being watched.

This joke takes a common piece of folk wisdom and twists it into a delightfully absurd scenario. The humor primarily comes from the subversion of expectation and the escalating paranoia of the joke's protagonist. We're all familiar with the age-old advice to count sheep when you can't sleep, a mental exercise meant to be boring and repetitive enough to lull you into slumber. The image of sheep, usually seen as docile and harmless, suddenly becoming watchful observers is where the punchline lands its comedic blow.

The act of counting sheep as a remedy for insomnia has been around for centuries, with some theories tracing it back to ancient shepherds who would count their flock. It's a classic example of a "mental floss" technique, where a simple, repetitive task distracts the mind from anxieties. However, our joke's character is already struggling with a feeling of being watched, and the sheep, instead of being a soothing distraction, become another source of their discomfort. This creates a humorous, albeit slightly unnerving, feedback loop that perfectly caps off the doctor's visit.