1. Your job isn't exactly making you happy
Being happy or pleased with your job are totally distinct states of being
2. Working long hours
Having a steady job does not entail working longer hours than usual
3. Negative office culture
Since you spend the bulk of your time at work, it's important to be surrounded by pleasant coworkers who make going to work bearable
4. Better employment prospects
Take advantage of greater career opportunities if they are presented to you
5. Health problems
It is a perfectly valid excuse to leave your job if you have ongoing medical problems