Stress
How is coaching stressful? Let us count the ways.
First, there's the game itself. It doesn't matter whether you're coaching a high school football team in front of a crowd of two hundred or a professional football team in front of a TV audience of millions. At every moment in the game, as you do what you can to guide your players to victory, your heart is going to be racing and your adrenaline is going to be pumping.
You aren't just watching athletes at the peak of their abilities try to out-compete each other; you're an integral part of that competition, an integral part of any win (or loss), so of course you're going to be keyed up.
Then, there's the fact that how your team performs—at least at the college and professional levels—often determines whether or not you get to keep your job.
Then, you have to constantly pray that your players stay on the straight-and-narrow, because there are some athletes at all levels and in all sports who are just terrible people, and having to deal with the fallout from a player's bad behavior makes it that much harder for you to do your job effectively.
Then, there's the fact that college and professional coaches tend to work something like a hundred hours a week and thus have difficulty making time for their families.
Then...well, you get the picture. Invest in a prescription for blood pressure medication if you decide to pursue a career in coaching, because boy howdy, are you going to need it.