You Can't Write Perfect Software
Did that hurt? It shouldn't. Accept it as an axiom of life. Embrace it. Celebrate it. Because perfect software doesn't exist. No one in the brief history of computing has ever written a piece of perfect software. It's unlikely that you'll be the first. And unless you accept this as a fact, you'll end up wasting time and energy chasing an impossible dream.
Everyone knows that they personally are the only good driver on Earth. The rest of the world is out there to get them, blowing through stop signs, weaving between lanes, not indicating turns, talking on the telephone, reading the paper, and just generally not living up to our standards. So we drive defensively. We look out for trouble before it happens, anticipate the unexpected, and never put ourselves into a position from which we can't extricate ourselves.
The analogy with coding is pretty obvious. We are constantly interfacing with other people's code—code that might not live up to our high standards—and dealing with inputs that may or may not be valid. So we are taught to code defensively. If there's any doubt, we validate all information we're given. We use assertions to detect bad data. We check for consistency, put constraints on database columns, and generally feel pretty good about ourselves.
But Pragmatic Programmers take this a step further. They don't trust themselves, either. Knowing that no one writes perfect code, including themselves, Pragmatic Programmers code in defenses against their own mistakes.
Software can't be perfect. Protect your code and users from the inevitable errors.