When all you have is a blowtorch...
I think it has a lot to do with the way the characters interact in the show. If the characters have chemistry, history, strong things in common, people are more likely to want to ship them.
In a lot of cases, the canon relationship isn't as strong as it should be or there are 'better' alternatives, so people ship them with someone else.
It's actually why I generally (note: generally, but not always) ship slash pairings.
Does that help?
"Yo, those kids are straight up liars, man. All I told them to do was run product. And by product, I mean chewing gum."
Good chemistry between the actors in question (and not with other actors), making sense within the plot, both characters actually being developed and real. Basically, not Shallow Love Intest, if it's a het pairing, the female character not just the smurfette, and so on.
X-ALL!
Since the question is popularity not quality, going for a less optimist view, Wish-Fulfillment may have a weight. In Total Drama Island fandom, there were a ton of DuncanxCourtney fanfics (once I did filter for this pairing and it returned 40% of fanfics were them), mostly written by girls and their fantasies of Bad Boy Turning Good. Or going for less pessimist view writing UST, Love-Hate relationships are fun to write and read, and some couples are more prone to it than others.
edited 24th Feb '13 3:50:04 PM by RN452
My work is here. Current main fic: Tengen Toppa Gurren Solvernia
I agree, I think the canon relationship has to make sense. For example, if they're Strangled by the Red String, most of the fans will root for another ship. They need to have more chemistry with each other than anyone else, and I think it turns out better if their relationship develops over time.
I've noticed that, while fanfic writers will 'ship anyone with anyone (and anyTHING), there are certain canon relationships that seem overwhelmingly popular with the writer base I see (e.g. it seems relatively rare to see Richard Castle with anyone besides Kate Beckett, and vice versa - compare with Kurt Hummel, who appears to be freely and frequently paired with every single male peer on the show, gay or straight).
What is it about a show or relationship that "cements" the canon relationship as the most popular?
edited 18th Jan '13 1:11:14 PM by Leaper