I've noticed that in multiple fandoms (Smallville, Stargate, X-Files, and Harry Potter for example) there may be discussions about plot, character, themes, and story, but in the end it all seems to boil down to ship. (Defining ship here in the broad sense - het or slash.) Sure, there are people in fandom that are there just for characters and plot. But those that are mainly focused on shipping is by far the majority. Once you weed through the hearty discussion of character arcs and plot, you'll see that people are arguing for "who ends up with who." Out of everything, this becomes the main focus and the most passionate for fans.
Take a look at any forum or discussion area. Doesn't matter which fandom. Most of them mirror each other.
So, I ask why? Why do we (in general) abandon everything else about the story, making it secondary to the ship? Why is it so vital to have the end goal be "so and so ends up with Clark" or "so and so ends up with Mulder" or "so and so ends up with Jack?" Why does that becoming the most defining attribute of the story or the characters? Why are we defining them by their romantic entanglements? Why are we defining them by their prize at the end of the day?
And is it the woman who is the prize in the end? Or the man? I've seen this go back and forth and it may depend on the fandom.
It happens across fandoms. I've noticed it a lot lately. As stargazercmc mentioned, is this something inate or does fandom perpetuate it? I wonder does this feed some basic human need or is something else going on here?