When asked to pick a number between 1 and 20, a preponderance of people pick the number 17. This is no mere spider-eating hearsay: the effect has been demonstrated over and over again, and discussed by such wry skeptics as Sean Carroll and PZ Myers. Humans are inexplicably inclined toward that particular number.
The best explanation is that 17 seems really random. Being prime, it isn't part of any simple arithmetic or geometric progression. And unlike the numbers 3, 7, and 13, it doesn't have any obvious cultural significance. Though the preceding arguments don't explain why 17 beats out 5, 11, or 19, it must be an attempt to seem unbiased and arbitrary that leads us to choose 17 from among a given range with thrice the statistical likelihood. What strikes us as the most random number is thereby made the least so.
Humans have such an affinity for patterns that we can't help including them when trying to generate random information - or overly excluding them as in this case. We are hopelessly non-random, and so since ancient times we've had to employ devices like dices, coin flips, and more recently, computers, to achieve random results for us.
Imagine for a second how different things would be if we possessed the ability to be random. At the beginning of an (American) football game for example, no coin would need be tossed to determine the receiving team; the ref would just shut his eyes for a moment, concentrate, rid himself of bias, and choose between the teams with complete fairness.
That is unimaginable. Our inability to be random - our inherent possession of preference - is part of what makes us human. But why such a preference for 17?