They say it's a dog's life.
They are definitely referring to stray dogs ("roadesians", as coined by my cousin).
That's because a pedigree dog lives life king size.
Consider my example. By the way I am a human being (fortunately or unfortunately I don't know ). And when I said consider my example, I meant consider my dog.
We are the most famous family of our apartment. And that is neither due to my parents nor due to me. The entire credit for our popularity goes to Velvet, my 7-month labrador.
Kids just love to get a glimpse of their four-legged celebrity. One day a few kids asked my father, "Uncle, what is his name?" (Velvet is a female. But normally people associate masculine terms with dogs)
"She has many names", said dad correcting the gender. Dad was right. I call her Velvet. Dad calls her Budi. Mom calls her by several names. In our house, Velvet and I have the maximum number of names. And at times it becomes confusing as to which of us is being called.
Anyways, the kids said, "We will call him Jimmy".
So one more name for Velvet. But she doesn't like this name. Might be because of its masculine flavor.

Velvet's day begins with food and this food part continues till she goes to sleep. She doesn't like eating dog food much (considers them as snacks, not as main course), so she eats bread, rice, chapati, milk, vegetables, meat, eggs, fruits...
And she expects them right on time.
And she gets them too, bang on time.
She sleeps on our bed. And in case one of us is also sitting or lying there, she will sit on top of us. And believe me it is not very comfortable.
She doesn't need to do any work. The only thing she excepts is lots of food and love. And she gets both in ample amounts.
For her my dad is her dad, my mom is her mom, and me her sibling. So in me she finds her playmate.
Right now she is sleeping peacefully on our sofa while dad is watching TV.

So people who say it's a dog's life must be referring to a very comfortable life.