Why noting them down must be a comfort to you.
This construction is a little odd to translate into my native language. 'Why' as a beginning word is normally used in interrogative sentences, but it is followed by a gerund. What is another simple construction of this sentence conveying the same sense?
I think a punctuation mark is left out.
Why, noting them down must be a comfort to you.
Why! Noting them down must be a comfort to you.
It's an exclamation (7) used at the beginning of a sentence to display an emotion (usually, surprise or anger).
The author doesn't seem to follow some punctuation rules.
"And it is not only me--I should say I," I continued; "I don't want you to run away with the idea that I am the only good man in the world. That's what I like about Christmas, it makes everybody good. The lovely sentiments we go about repeating! the noble deeds we do! from a little before Christmas up to, say, the end of January! why noting them down must be a comfort to you."
The interogative pronouns "what" "how" "why" etc can form content clauses meaning "the thing that...", "the method that..." and "the reason that..."
So the apparent meaning is
(The reason that you are noting them down) (is assumed to be) (a comfort to you).
However this is junk. It is very unlikely that a reason for noting something down could be a comfort.
But I can use the same construction in a reasonable sentence:
I will tell you why noting down the speaker's comments can help you in a debate.
Which means "I will tell you the reason for you to note them down."
