It is not unnatural (as Tom Jones didn't sing)

This week I was completely baffled by a textbook reading. As such, I've been spending a lot of time today trying to puzzle out the intricacies of this page. Perhaps too much. I ended up trying to do a full-on analysis, which is hindered by never having really studied syntax, let alone Japanese linguistics. However, I did end up with a complicated table that I thought I'd share. Maybe someone will appreciate it.

The brackets (「 and 」) indicate the start and end of distinct grammatical elements. I've put things in different columns to indicate how deeply nested a particular phrase is.

					English	Grammar
「	「	「	「 それが		This	NP
				「 普通だ 」	Is ordinary	VP
			と思っている 」		Are thinking	VP
		「 日本人 」			Japanese people	NP
		の			Of	PRT
	習慣 」				Custom	NP
からすると 」					From the perspective of	PP
「	「	「 その時 」			This time	NP
		「 一回かぎり 」			Once and for all	AVP
		「	「	「 お礼 」	Politeness	NP
			だけ 」		Only	AVP
		で 」			By	PP
	は				(topic)	PRT
	もの足りなさ 」				Insufficiency	NP
	を				(obj.)	PRT
「 感じる 」					To feel	VP
	の				this case	PRT
	も				(Emph.)	PRT
「 無理はない 」				Is not unnatural	VP
」

From the point of view of the cultures of the Japanese people, for whom this* is usual, it is not unnatural to feel the inadequacy of only a single expression of gratitude once-and-for-all on this occasion.**

* that is, repeating your thanks on several occasions

** that is, having done someone a favour

More naturally: From the point of view of the cultures of the Japanese people, for whom this is usual, it's natural to feel that thanking someone only once for a favour is insufficient.

I hope, glancing at the nested clauses and intricacies of this particular sentence, you can have a little insight into what I'm doing, and perhaps feel a smidgen of pity. Right, back to work...