A line from a song lyric: "人理所当然的忘记"

Question

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 & 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 "a decent person should not forget (someone who's so devoted)". How does it work?

Answer 1

A military song perhaps?

理所当然: as it should be by rights (idiom); proper and to be expected as a matter of course / inevitable and right

也许当时忙着微笑和哭泣
We didn't know there was anything else to do in the world except smile and cry,
忙着追逐天空中的流星
busy chasing falling stars in the heavens
人理所当然的忘记
inevitably we forget
是谁风里雨里一直默默守护在原地
(those) who, ever silent, ever still, stand guard

Forgot the rain and wind:

(those) who, ever silent, by wind or rain, ever still, stand guard

Answer 2

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