A line from a song lyric: "人理所当然的忘记" This is a part of lyrics from the song 小幸运 by 田馥甄:

也许当时忙着微笑和哭泣
忙着追逐天空中的流星
人理所当然的忘记
是谁风里雨里一直默默守护在原地

I would like to know how the third sentence works out, in terms of grammatical structure. The surrounding lines are for context. Here's my progress:

也许(maybe) 当时(at that time) 忙着(busy) 微笑和哭泣(laughing &amp; crying)
忙着(busy) 追逐(chasing) 天空中的流星(falling stars in the sky);

The two sentences above is like an adverb (i.e. is separable; does not affect the following sentence grammatically), I suppose?

人理所(???) 当然的(something that is required) 忘记(forget)
是(is) 谁(someone who,) 风里雨里(in hard times) 一直默默 守护在原地(kept one's place without saying anything)

I think 人理 is used here to convey such as &quot;a decent person should not forget (someone who's so devoted)&quot;. How does it work?
人 - people (subject)


理所当然的 - naturally (adverbial phrase)


忘记 - forget (verb)


理 (logic)

理所 (according to logic)

当然 (of course; certainly)

理所当然 = naturally; of course

理所当然的 = naturally; of course