The couples that form, the people who get married, this pairing up often occurs as a consequence of circumstance. After numerous girlfriends, he may say that the one he is currently dating is the one, when really, the reason he’s going to marry her is more because he’s almost 30 and time is running out than that he likes her more than any of his previous partners. This happens on a lesser scale, too. I’ve seen friends take each other’s girlfriends/boyfriends when their original mate was away or at the hospital. It has something to do with the natural response to being around someone of the opposite sex more than anyone else of the opposite sex. If c is around d more than any other d, and d is around c more than any other c, well then, c & d hook up, regardless of c & d’s prior commitments to other letters.