Well, it really depends on your management style. OT1H, you might want your staff happy, so they run on autopilot and you can thus completely forget they exist, allowing you to focus on other things. But OTOH, at the bottom line, your objectives are to make peeps, happy, not employees, and to take money from peeps, not give it to employees. So you can alternatively hire 'em cheap, work 'em 'til they won't, and replace 'em. It just depends on how draconian you are and how much time you want to devote to HR
But all that aside, in general employees require 2 things be REASONABLY happy: a "normal" workload and pay proportionate to their training level. Or alternatively, a stupidly high salary regardless of workload--you can buy their happiness if you're willing to. I also say "reasonably" happy because all employees have their own personalities. Some are go-getters, some are slackers, some think it's an honor to work for you, some just have attitude problems and/or are clinically depressed. So while you can usually make most of your staff happy most of the time, you'll always have a few soreheads until you weed them out through attrition and replacement.
Training decreases workload by making the employee more efficient. So in general, an employee should only be trained when he's got a "high" workload. Training will reduce this to "normal". But OTOH, a high workload is usually a symptom that something else is wrong with the park. If a shop vendor has a "high" workload, then you probably need more similar shops in that area. If a janitor or mechanic has a "high" workload, you should adjust your work rosters to spread the load more evenly, or maybe hire another janitor/mechanic for a given work roster. But if a guard has a high workload, you've screwed the pooch already--that should never happen. You're not making your guests happy enough and/or don't have cameras covering ever square inch of the park. Guards should always have "low" workloads.
Any time you train an employee, you should also throw in a pay raise of about 10%. This is cheaper than hiring another employee to share the load but, in general it's better to fix the park problem that is creating the high workload that you want to train the employee to handle.
If you keep your employees at "normal" workload and paid to match, they'll USUALLY be REASONABLY happy, as in they won't quit. All employees will quit when they feel over-worked and under-paid. All employees will stay working if they feel under-worked and stupidly over-paid. Some impossible-to-please employees will quit anyway even with normal workload and market standard pay. The lesson here is, NEVER throw money at morale problems. If the guy's got a "normal" workload and adequate pay, and is still getting unhappy, he's a loser. The sooner he quits, the better. Then you can replace him sooner and hopefully get somebody with a better attitude.
HOIWEVER, be advised that shop vendor workload is HIGHLY dependent on guest preferences. Peeps have very strong preferences so some types of shops will never get much business while others will be slammed. In general, it works like this:
Food: Love burgers, really like hotdogs, pretty much hate everything else.
Drink: Love water, really like slush, pretty much hate everything else. And if you've got benches all over the park, you'll sell VERY little energy drinks and zero coffee.
Gifts: Love balloons, really like hats, hate mementos.
So, it doesn't pay to give peeps a balanced diet or wide menu choices, because they don't want anything but burgers and hotdogs, water and slush. They will only consume the other food and drink if there are no other choices, at least within reasonable walking distance. Therefore, if you've loaded up your park with unpopular food and drink shops, their vendors are NEVER going to be happy because they'll have "low" workloads even with zero training, and will be quitting constantly. Best bet there is to bulldoze those shops or change them to something peeps actually like.
Likewise with gift shops., A big, edge-to-edge park with no limits on peep numbers can support 3 balloon shops, 2 hat shops, and 1 memento shop. Any smaller park cannot keep a memento shop open and should reduce the others to 2 balloons and 1 hat. Very small parks should only have 1 balloon shop. Having more shops than this will just be futile as the vendors will be quitting constantly, unless you pay them huge amounts to stand there and do nothing all day.