> The best way to get the right answer on the internet is not to ask a question; it's to post the wrong answer. source 
A supposition about human nature, i.e. that people would rather argue against an assertion than fill in the blank. Given that it’s claimed to have bene coined in the early 1980s, it’s likely it was phrased in terms of Usenet forums or similar contexts rather than more modern formats like blogs. The date of the law is highly uncertain, since it’s been described second-hand and was a casual aphorism rather than a formal publication.