Dating someone in your profession. You only do 2 basic things - eat and shit. Shitting's done in the bathroom. If you're eating in the bathroom too, then you'll never leave, just like if you're dating someone in your profession, you'll never escape your job.
Dental Student #1: It's official: I'm dating that girl in our class.
Dental Student #2: Ah man, you're shitting where you're eating. Get out while you can.