So, how did A meet F? It's 'cause they both grew up in the Bay Area, right?
Well, it's not quite that simple; actually, they met at an event at D's place. Because D had recently started going out with E, and F was a college friend of E's.
So the chain goes A, D, E, F. Simple, right? Straightforward.
But no: A originally met D through C. See, A knew C in high school, and C knew D in college. (The same college, incidentally, where E and F knew each other, but C and D were there a few years earlier.)
So A and C went to high school together? Actually, no. A went to high school with B; B and C had been friends since they were kids; B started bringing C to social events even though C went to a different school.
So the chain goes: A was in a social group with B during high school. B knew C from when they were kids, and introduced C into the high school social group. Much later, C met D in college. After college, C and D both ended up back in the Bay Area, and some time later, D and E met and got together. And E had known F in college. So eventually D invited A, C, E, and F to this social occasion, and that's how A met F.
So in a sense, the fact that A and F ended up together is due to B; without B, there would've been no connection to C, and thus no connection to any of the others.
(As an aside, C has a good track record for introducing me to cool people; this is far from the only connection-chain that C is essential to.)
(As another aside, I think it could also be argued that I was instrumental in all of this, but in a more oblique way than any of the other mentioned letters; I didn't actually introduce anyone to anyone else, it's more just that I was part of the social glue that resulted in some of those people spending time together.)
But if you examine each of the links of that A/B/C/D/E/F chain closely, no doubt there are other links. How did A and B both find themselves in that social group in high school? Why did B's parents know C's parents when B and C were kids? Who introduced C to D? And there are surely several missing links between D and E, and between E and F; I don't know anything about those.
I find this stuff fascinating. (Though I realize it loses something in the translation into alphabet soup.) Connections that were people's first introduction to each other, forming a chain from one person to another, jumping forward and backward in time. What links, if missing, would have resulted in couples never meeting? What hidden links are only discovered later? And what granularity makes sense when you're trying to figure out who introduced whom to whom else? It seems like a fractal to me: every time you look closer, you see smaller-grained patterns hidden in the larger ones.
The chain of connections between me and Mary Anne is one I particularly like, because it's a loop. In one direction, my first-pass list of links gives four people whose connections had to be in place to result in M and I meeting; in the other direction (totally independent of the first) it's five people. (That's just the people I know about, mind you; there are at least a couple of links in there where I don't know who introduced two "adjacent" links in the chain.) And it was an unusual circumstance that led to our first meeting (through one set of links), and another unusual circumstance that led to our second meeting (through the other set). Someday perhaps I'll tell those stories. For now, am just musing about the likelihood that if any one of those links hadn't happened, M and I probably would still have met (due to the other set of links), but wouldn't have gotten to know each other nearly as well or as quickly as we did, if at all.