The root "obrigad" sounds a lot like the English word "obligate". If this is more than just a coincidence, then a more direct translation is "I am obliged" rather than "thank you". Said this way it makes a lot of sense why the verb is determined by the speaker.
As an aside, I don't think Spanish has a word that's quite like "obliged". Maybe "endeudado", but that's just "in debt" and is not particularly about favors.
grammatical gender of a person == physical gender of that person
But compared to the differences in, e.g., Japanese that some people here are discussing, this seems rather minor.I am handsome.
I am beautiful.
I told my girlfriend she was handsome the other day and she objected strenuously.
https://english.stackexchange.com/questions/355908/why-is-ha...
Apparently not:
I did it myself: Eu fiz eu mesmo/Eu fiz eu mesma.
And most adjectives. Actually, "obrigado" is "obliged" so the phrase "much obliged" is equivalent to the Portuguese "muito obrigado".