I’m going to take into evidence all canon shows and movies, if you want to argue an alternate take going off of only TOS or whatever cool I’m just stating my sources.
The pilot of Enterprise shows us the first contact between proto-Federation(mostly Earth humans) and the Klingon empire.
A badly injured Klingon who came to earth by chance while fighting another alien is nursed back to health and returned to the Klingon homeworld, against the advice of the Vulcans. The facial expressions of the Klingon mucketymucks says it all, the humans have just made a gigantic cultural faux pas and don’t even realize it. They have done something really shocking and disgusting by cheating him out of his honorable death, and nursing him back to health only to then dump him in front of his leaders, and they weren’t even the species the Klingons were fighting just clueless bystanders basically. The humans think they scored brownie points by this too!
Even to Kirk’s time the Fed just doesn’t seem to be able to grok the Klingon mindset, they just don’t! Even to the fifth TOS film we have a member of the Klingon delegation saying that saying all aliens have human rights is preposterous.
Is that the cause of the whole Fed/Klingon conflict? The humans flat refusal to believe an alien species has very different cultural ideas?