Well in my opinion Avatar (The Last Airbender only, not Korra) has a great story and executed nicely. They did throw in a few moments of fan service moments just for laughs every once in a while unlike Legend of Korra, where the romance between characters took the spotlight and it got quite tiresome. I think what drew people to TLA would be Aang's journey to be an Avatar and discovering the world of Avatar. Korra's story felt really rushed (since it's a mini-series). Hopefully, it would get better for the next few seasons.
I guess you could say Twilight is popular because of shipping, but I feel like it was because of how fans are obsessed with the male characters. The story is horrid.
There is no need for official pairs/shipping, but I guess it happens when fans are passionate about what (or who) they like. It happens in everything. Heck, people ship real life people too. In my opinion, if a story goes on relying just on the fan shipping aspect and doesn't change, people would eventually get bored of it. Example: The US version of The Office. At the start, many were rooting for Jim and Pam to be together. Fast forward a few seasons later,
Sorry, please read the forum rules to see why you can't view spoilers and why you can't post in this forum section. Thank you! and a lot of people complained that they got boring and couldn't stand them together. :sob: You just can't please everyone.