That's funny!

Low pay, no benefits, and never a complement from the boss. But once I struck out on my own and hired a dog, it's much better. They work for almost nothing, accept pats on the head in lieu of benefits, and always say you are a great boss.

When we had dogs I got more exercise and spent a lot more time outdoors. We went on walks and interacted with people who wanted to pet my beautiful doggies. We went out to play fetch several times a day, or I would sit in the sun and read while they sniffed around.

After my dogs passed, having only cats made it easy for me to become a recluse. I adore my cats but overall, I think my life was happier with dogs in it.

Studies show that moderate exercise is as effective a treatment for mild to moderated depression as medication. Then, as you noted, throw in the social interaction piece.
On edit
I am closely supervised by 3 cats, one of whom is 18 years young.

Means nothing to this 55 year old happily married guy. Arlo likes the pets though.

Should have got him when I was 16.

An old (single) roommate often offered to walk my lab for that very reason.

their cats had just ralphed on freshly shampooed rugs.

and introverts don't always express happiness in the same way as extraverts. As an introvert with cats, I'm happy just to be left alone, and I'm not at all unhappy in general. Also, cats, being relatively low-maintenance and inexpensive as compared to dogs, are often the owners of people who are elderly, who live alone, or who have less money, and therefore might not consider themselves to be as happy as people who aren't old, alone or poor.

follows me everywhere, is always next to me/trying to snuggle up next to me, hates when I leave in the morning and goes nuts with affection when I return from work at night.

Other than the walking outside bit, there's nothing a dog would bring that's better.
And I like dogs as much but as a single guy who works, I don't want to leave a dog alone all day.