Stress
You know when you get up in front of an audience to give a presentation or act in a play or participate in a team sport, and your hands get clammy and your stomach starts to tremble and you suddenly have to pee? That's what being a marketing manager can feel like.
You'll be working with clients or employers who will demand you give a project your all, and then some. You'll have to deal with deadlines that will leave you zombified from lack of sleep. You'll also be operating with the knowledge that everything you do at work will have a direct (and—fingers crossed—positive) impact on your company's revenue stream.