The answer is simple: forget things you need to take with you.
I was on my way to the library, when I realized that I'd left my library card at home. I was suitably scolded for this transgression during my last visit to the circulation desk and I did not wish to experience a reenactment. So I went home and got it.
Upon arrival at the library, I found a space at the rack and then discovered that my lock was in neither of the panniers. So I had to turn around and go home. Now I have to get ready for dinner at a friend's house and there's no time to go back. No big deal. The books aren't actually due back until tomorrow, which gives me an excuse for another ride.
I'm a little frustrated, sure. But if I had driven instead, I'd be even more annoyed. I'd have wasted gasoline and still wouldn't have nothing to show for my efforts. On the other hand, my reward for being forgetful on a bicycle trip was an enjoyable 5 mile ride on a sunny Saturday afternoon.