I'm on decaf now. What I miss most is the road rage.

Letterman’s joke hinges on an incongruity: switching to decaf is usually framed as a healthy, calming choice, but he claims to “miss” an unhealthy byproduct—road rage. The humor comes from treating irritability and aggression as if they were pleasurable perks of caffeine, and from the self-mocking implication that his temperament was chemically fueled. It also satirizes modern driving culture, where stress and anger behind the wheel are common, and suggests how people can become attached to their own bad habits when they’re familiar or energizing.