Why a dog or donkey is most of the times used as a synonym for wrong , ugly and bad qualities in humans?

I fail to understand why a dog or donkey is most of the times used as a synonym for wrong , ugly and bad qualities in humans. If humans could ever reach the standards set by these animals in trust, love, loyalty, discipline, integrity and hardwork, the world would have used humans as synonyms for all these traits.

We still say ‘ Work like a donkey’ or ‘As faithful as a dog’. And these animals do all this acknowledging only the food and shelter given to them. Any human to do the same would definitely expect a fortune in return or never ending appreciation.
So do we really belong to the superior species?

-Sucheta Gour