Well not entirely true. To make a Pokemon happy you need to:
1. Walk a lot, though Joy is a mindless robot who stays at her permanent location.
2. Win gym battles with your Pokemon, though it's kinda hard with a level 1 Chansey who barely knows Softboiled.
3. Make your Pokemon a haircut, though the Joys don't even know what a haircut is, because they're nothing but evil clones secretly made by Mewtwo in the first movie.
4. Feed your Pokemon with Vitamins. Now, taking in mind Vitamins are really expensive (10,000 zenny per vitamin? Damn, that's quite expensive), and the Joys barely make money, that's quite hard.
Why do the Joys barely make money?
4.1. It's really hard to make any money when random 10 million people appear, earn the whole money in the region, then quit the game. What a way to ruin the NPCs' economy.
4.2. Imagine working with a shop, that exists everywhere, in every damn single city, and you can't say you're better than another branch. Basically, imagine McDonalds.
5. I don't really know, but if I was a Pokemon, I could become REALLY depressed if my trainer's name was "Joy", while I'm living in the "I just got really high" land (I mean come on, I bet the idea of Pokemon was made when the author was really high. Hell, the idea to make Nintendo, or a handheld console, was made at the same situation).
So long story short, just gameshark Sacred Ash, you don't want to see Nurse Joy getting mad after she sees you EVERY. FIVE. MINUTES. (in case you're a really bad Pokemon player).
By the way, you know what else makes a Pokemon depressed? Any try-hard-jokes-in-the-name. True story.
Oh and, I think I know how the Joys always heal the team so quickly; they just found an easy way to get infinite Sacred Ash's (gameshark anyone?)