It is a matter of preference, however, in my tentative opinion, I think Kanji-san's answer is more appropriate and useful in the end. Because the best answer here saying "It is all about the matter of the subject", explains "the effect" part of "cause-and-effect" while Kanji-san's explains "the cause" part.
Why "the one of the former is usually a person"? It is because 片付ける is a transitive verb. And why "the one of the latter is usually a thing"? It is because "片付く“ is an intransitive verb.
I understand that "simple conclusion without reason" style of the answer is sometimes more fascinating for leaners at a beginner level. I believe, however, it won't work in the end when you get into middle level.
お掃除ロボット（a machine/ a thing)が部屋をキレイに片付けてくれた！
このひどい計画(a plan )が、俺達の夢をすべてどこかへ片付けてしまった。
（It is a English-translation-like Japanese, but it is grammatical and works)
ウチの娘( a person) もやっと片付いたよ (Finally, she got married)
Or you cannot understand 加える and 加わる with "thing-person subject" method.
But those lines can easily be understood by "transitive-intransitive" theory. I am sure the logic will take you further.