Why do people often say "it's really a dog's life"? What do you mean?

"Quite a dog's life" is a slang term, which is usually used to describe someone's great vitality or great luck. It doesn't literally mean "live like a dog", but comes from the English expression "lucky dog", which means "lucky dog".

This expression is usually humorous and witty, so it is often used in relaxed social situations. However, it should be noted that this expression comes from informal spoken language, so it should be avoided in formal occasions.