Alice dates Bob. Bob is friends with Dave. Dave is single. So is Alice's friend Carol. The Rules of Fiction™ dictate that either Alice and Bob break up or Dave and Carol get together. There is no other way around it. Can go on to include larger cliques; so long as there's an even number, no one will be single or date outside their clique, since we can't have them date outside their clique.
May [...] [tvtropes.org]