Why is Charlie Brown saying good grief in the great pumpkin?

Answer 1

He says it frequently in the comic strip -- it's his tag line. He says it whenever things go wrong or become confused for no apparent reason, or when other characters do things that defy explanation.

Answer 2

Charlie Brown says "Good grief" as an expression of frustration, disappointment, or disbelief, often in response to the antics of his friends or the situations he finds himself in. It is a catchphrase that reflects his perpetually exasperated and slightly anxious demeanor.

Answer 3

If the context you are talking about it in, is the English language then alternatives would be 'Oh My Goodness' he is exclaiming it.