In Japanese, the subject seems to be reversed in passive sentences:

• ケーキが私に食べられる (I ate the cake)

This is because the idea of “passive” is different in Japanese. You should think of the “passive” not as an agent doing something to an object, but rather an object receiving an action:

• The cake was eaten by me -> The cake (Nominativ) received the action of being eaten by me (Dativ)

Therefore, you should think of the “passive form” of verbs not as this special form, but rather as a new type of verb, called receiving verbs. Conjugating the “passive” form turns a verb into a receiving verb.

Building §

append the passive helper verb to a verb

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From Unlocking Japanese:

In Japanese, the linguistic term for the passive construction seems to be ukemi. The kanji for this mean “receiving body”. So from this, one could say that in a passive construction the subject is receiving an action. I do not think that one could really argue, in the sentence “ I am receiving a gift”, that “I” is not the grammatical subject. What am I doing? I am receiving.