Why do people sometimes mourn the death of a pet as much as they do if a loved one were to pass?

Obviously, you have never been a pet owner, or you wouldn't be asking such a question. For those of us who have been and still are pet owners, to us, pets are like family. The grief we experience when a pet passes is as real as the grief we would feel at the death of a person we care about. And it isn't a question of sometimes mourning a loss of that type. Many pet owners never entirely recover when an animal they love dies. I personally have seen grown men break down and cry like a 5 year old with a scraped knee because a dog or a horse they loved is gone. It doesn't make them less of a person- it just means they're human like we all are.

These are true nature lovers with a big heart.

Because the really good pets are there for you no matter what, through thick and thin. That is more than I can say for non-family humans.

Because pets, especially dogs, often provide unconditional love, and it is hard when such a creature goes.

Because pets don't beat me like my wife and children does

Because they are stupid. Those are usually the tree huggers.

Because the pet is a loved one, sometimes a loved one that they have spent far more time with and liked more than the loved one that passed.

Because pets are loved ones.  Pets provide unconditional love to their owner, unlike people.

If you have never had a pet it’s hard to explain. 

people attach themselves too closely to animals