Running into the person who fired you is kind of like running into an ex (the one who dumped you, of course). You ideally would look your best-10 pounds lighter, having a great hair day, wearing your hottest new outfit. You would have a fabulous new job (or relationship, or both) to brag about.
Unfortunately, you usually run into your ex when you're sweaty, haven't bothered to brush your hair, and are carrying a load of laundry and perhaps a pint of Ben & Jerry's Chubby Hubby. You probably smell.