6.) A friendship doesn't form between two members of the opposite sex without one of them having feelings for the other at one point.
Describes the the common phenomenon regarding relationships between two members of the opposite sex. Proven true time after time, it has caused heartache of many men and women through the centuries. The abbreviation "Rule #6" comes from a list of advice posted online where this particular piece was 6th on said list.
"Carly has feelings for me, but we're best friends- I don't want to be anything more. I guess Rule #6 is striking again."
Person one: "Hey, where's Max?"
Person two: "I don't know, we're not everywhere together all the time."
Person one: "It seems like it, you better be careful before Rule #6 strikes him down."