When Zeus created human beings, he realized they were not perfect. Far from perfect, in fact.
"They'll need sacks to carry their faults in," he decided, and so he created some great big sacks, enough so that every human being would have two sacks: one sack to carry their own faults, and a second sack for faults belonging to others.
Zeus threw the sacks over each person's shoulders: the sack with other people's faults hangs down in front, and the sack with their own faults behind.
So that's why you are more aware of other people's faults than your own.
Inspired by: Fables of Aesop and Other Eminent Mythologists by Roger L'Estrange, 388.
Notes: This is Perry 266.
Here is an illustration from a 17th-century edition of Phaedrus: