I went to the doctor because I felt like I was losing my memory.

This joke gets its chuckle from a delightful dose of irony, where the very solution offered to a problem is immediately undermined by the problem itself. The setup presents a relatable situation: someone concerned about their memory. It's a common human experience to occasionally misplace keys or forget an appointment, leading many to seek a doctor's advice.

The humor then stems from the doctor's perfectly sensible advice – to jot things down – which is a practical tip often given for minor forgetfulness. However, the immediate failure to follow this advice, simply because the patient forgot the essential tool (the pen), brilliantly illustrates the severity of their memory issue. It's a comedic loop that highlights the frustrating and often amusing cycle of forgetting.

The punchline works by creating a sudden, self-inflicted obstacle that proves the initial complaint without needing to say anything more. It’s a classic example of how a character's actions can comically demonstrate their predicament, making us laugh at the relatable struggle of a mind that just can't seem to hold onto information, even for a moment.