...If you cry at work multiple times a week purely because of work and not because of hormones or the fact that you just watched an episode of Parenthood, you should probably quit your job.
...If you stay up really late at night because you know the sooner you go to sleep the sooner you have to go to work and you just want to put it off as long as possible, you should probably quit your job.
...If the employees at 7-eleven make more money than you per hour and you would rather be refilling slurpee machines for half what you make now because you hate your job so much, you should probably quit.
...If you're afraid to complete any task given to you because you know your boss will find something wrong with the finished product no matter what, you should probably quit your job.
...If your boss does nothing but criticize you and blame the company's failures on you even though you're not responsible for any of it, you should probably quit your job.
...If your job makes you an angry, cynical, bitter person, you should probably quit.
...If you've ever said the words, "My job is sucking my soul out of me," you should probably quit.
...If your job is not fulfilling you or making you feel good about yourself and the work you're doing, you should probably quit.
...If your boss has cut your hours down to 8/week, you should probably quit.
...If your boss sends you emails at 4am and expects them to be read and responded to before you've even gotten to work, you should probably quit your job.
Obviously you shouldn't quit your job if you have nothing else lined up. I did that once. In the middle of a shift I went to the parking lot with the assistant manager, sat on a curb, and sobbed about how miserable I was. I never went back. The next couple of months sucked royally but I will always feel that quitting that job was one of the best decisions I ever made.
The job I have now is stressful and busy and exhausting, but I love it. The people are great, the work is fulfilling, and I never dread going to work. If you hate your job, if your job is making you miserable, if you dread Mondays with the white hot intensity of a thousand suns, change your circumstances. Just do it.