それこそ 僕に与えられた使命だと思っています。
I think that is why I was chosen for the errand.
Where did the why translation come from?
My translation would be: I think that is truly/for sure the duty/mission/errand that was given to me.
I also asked ChatGPT and the translation given was :
I believe that is the mission given to me.
Both translations, mine and ChatGPT, mean very different to the translation Bunpro has.
I think it is coming from the 与えられた being in the potential form. If it was just the mission given that would be 与えた使命. 与えられた使命 is the mission that was able to be given to me. それこそ 僕に与えられた使命だ = for that exact reason, it is a mission that was able to be given to me. then you add the と思っています to make it “I think.”
The reason why you would tell someone why you think a mission was given to you is because you are giving an explanation. I think the mission was able to given to me for that exact reason. So the why comes from context of this sentence itself being given as an explanation.
If you add in the context of someone saying this as an explanation, you get “I think that is why I was able to be given the mission” or “I think that is why I was chosen for the errand.”
I think neither you, nor ChatGPT and Bunpro are wrong here. The sentence is formulated in such a way that given proper context it could mean all of these things. However I understand the confusion, but I cannot help you otherwise.
That would be: それこそが僕に与えられた使命だ…
But それこそ can be used adverbially to add emphasis. Maybe the context here is that before this sentence, they discussed the reasons, and the translator decided to translate the emphasis as “why”?
…maybe it’d be a better example sentence if it was the other one with the が, though. But also, それこそ shows up as its own thing in dictionaries, so it’s probably easy to accidentally use it in a way that doesn’t 100% match the grammar point.