Stress
Have you ever had that nervous tingle in your belly when you're on stage for a class performance? Or standing in front of a bunch of people giving a speech that you only just finished twenty minutes before? Or when it's three to two in the bottom of the ninth, and that cute boy or girl you've been trying to impress is sitting just beyond third?
Imagine all of that, only you have to a hit a note as high as the moon in front of three thousand people, all of whom paid up to $2,000 a ticket and are judging you like the tabloids judge a reality TV star. That's what you'll be doing for work every night for the rest of your life. And you'll have to rehearse twelve hours a day, six days a week, forty-two weeks a year—if you're lucky.
If you're not lucky, you'll be spending a good majority of your time practicing and learning and rehearsing and rerehearsing and auditioning and reauditioning just to get a chance to do it all again next year.
All that doesn't even account for the various personalities you'll have to deal with. Dressing rooms are notorious for being crowded, noisy, gossipy dens of talented narcissists, and the occasional diva (which is used here as a gender-neutral term for a person who takes over a room whether you like it or not) can multiply the frustration.
The only reason it works is because these same artists often become like your family—even if there's occasionally that odd cousin you only care to see every Thanksgiving.
And if that wasn't enough stress for ya, try doing it without actually being able to sing. It probably isn't going to work out very well.