There’s also some difference in thought processes. Studies show that dogs understand things like “fair,” a dog will get upset if another dog gets a treat and they get none. However- these same studies indicate that dogs don’t understand inequity. If you give one dog one treat and another dog two treats at once in view of the other- the dog that received less doesn’t display the signs of being wronged as if they had received no treat. They seem to be fine as long as they get something, even if it’s not equivalent.
What’s more- dogs tend to trust things more, trust people more. If you’re kind to a dog it’s likely to assume you will continue to be kind to it. Even if you treat it cruelly it is likely to assume that’s an exception and not the rule. Cats and many birds however tend to discount kindness for even a single instance of sufficient perceived cruelty. Humans don’t trust humans so implicitly as fogs because... well, humans do messed up stuff and we are well aware of that. We know humans can use kindness or lies to mask ulterior motives and such.
And the people have bad manner by choices.