In ancient times, just as today in third-world societies, dogs lived miserable lives with little or no human care.
This led to the expressions for hard-times such as, “it’s a dog’s life,” “sick as a dog,” and “dog-tired.”
As for the proverb “Every dog has his day,” it was first recorded as an epilogue after the famed Greek playwright Euripides was killed by a pack of dogs in 405 B.C.
The common interpretation of this expression is that everybody has an opportunity for a moment of glory.