Stress
Besides the typical stresses on your body, there are plenty of other stresses for fitness instructors to deal with in their daily lives. First off, whether the job is at a massive fitness center or a local YMCA, you'll be working with a lot of people. This means you've got to be able to handle all sorts of different personalities. Some of your clients will be accustomed to a general "I don't wanna" level of effort that you have to work around. Others will be overachievers who are dead-set on showing you how good they are―and the worst of the worst will kindly point out any mistakes they see in your form.
You've also got a lot of body types to deal with, and it's important to know how far you can push some before they start pushing back. The beauty of fitness is that people are working towards a goal, and it's a goal that you can help them set and ideally achieve. Unfortunately, this means you may get sworn or yelled at every once in a while. Do what you tell them to do: Channel it into motivation.
There are also side benefits to this career lifestyle that can help ease some of the stress involved. The first and best is flexibility (schedule-wise): Fitness instructors pretty much make their own schedules. Sure, their place of business may have set classes that are mandatory, but doesn't have to be the nine-to-five grind that many of your clients will be crawling through.
You've also got a great opportunity to meet people from all walks of life, who can often bring rays of sunshine to a somewhat miserable day―or maybe just sound financial advice, which is helpful in its own way. You also get to pick the music, which is suh-weet.
And of course, if you're having an overly stressful day, at least you know of a place to go that offers meditation classes.