Here are some questions I ask you, and you don't have to answer them all (though that will be nice if you do).
1) When you write the shipping your story centers on, why do you do it? Is it to attract like-minded people to your story? Is it to gain more attention with a specific pairing rather than to have your story be a gen fic?
2) If you do not state a pairing in your FFnet summary, and you receive reviews supporting pairings you did not specify nor emphasize, how do you feel about it?
3) What are your experiences with readers when you ship or nonship a fanfic?
4) Say you come across a fantastic story that has no stated/confirmed pairing in the beginning. As you read more and enjoy it more, you reach a turning point in the story where a pairing you do not prefer is confirmed. "Three Pieces of an Annoying Complex Puzzle" is one such example. Do you continue to read it?
I remembered talking to a reader one time (and only once) and what she said. She told me, truthfully, that when she first read about Naruto and Sakura's tongue dance, she was about to stop reading. But then, when she saw that Hinata was described next and she read the following interaction between those two, "a little flame of hope lit up." So she kept on reading. Then, I said, "So you kept on reading for the N***H***?" She amended by saying, "Well, I also liked the story..."
Secondly, I wrote a one-shot involving Team 7 friendship. Using the shipping system as a way to explain that the interactions between the three are strong and important, I said, "You will be seeing S***S***, N***S***, and NaruSaku pairings!" ...I got flamed by a very pissed-off yaoi girl.
"Don't claim to have pairings when you don't. You said N***S*** and I was expecting N***S*** but all I got was S***S***. And I utterly loathe het. Oh, God, I hate het writers like you who try to trick people into read their stories. You blame us yaoi writers about lying and cheating you, but you het writers do it too."
Needless to say, I learned my lesson.
Thirdly, I received a FFnet message from a fangirl who asked me quite bluntly, "Are you a boy or a girl?" When I asked her why she gave me such a question, she replied that, "Most girls write romance, but your story doesn't have any romance in it. So I couldn't tell."
How do you think these circumstances have contributed to my writing fanfics? These are only the more important factors of many I've experienced.