Its because of the difficulty of creating conflict to the story
and its also easy way to make reader experience emotion.
Possibly. But I prefer fluffy happy nice emotions to crying unhappy pouty ones.
On a related note, I prefer my shipping with a completely feminine bias. Reading Twilight say "Oh, yeah, baby! Take it! Take it!" just makes me shake my head. But a story like
Don't Let the Sun Catch You Crying, which is about 40 pages of totally unresolved sexual tension and emotional bonding, that works for me. It's sweet and cute, and it doesn't require me to wonder what happened to the ponies to act that way.
Again, I want sugar bombs of fuzzy happy goodness in my ponies. Not grimdark, not hardcore erotica, not psychokillers or nighttime terrors.
I'm a nitpicky bastard
I think my biggest complaint at the moment is that too many authors are trying to use the same devices. It seems like one in two fics has somebody say "It. Is. On." About one in three has a poison joke event. Every monster is a manticore.
I understand that fanfic writers are trying to draw from a limited pool of only 28 episodes so far, but they really don't need to. It reminds me of a similar problem from Star Wars fanfics: you have an entire galaxy with millions of habitable world, yet 90% of fanfics take place on one of the dozen or so planets named in a movie.
It's not necessary. It doesn't make for good reading after seeing the same devices so many times. My favorite fics have been those that take existing characters and put them in new circumstances. It's the
characters we love. Drop Pinkie Pie in the middle of the desert and have the locals decide to sacrifice her to their local deity and have her misunderstand and try to make it a fun and exciting event, while everyone else goes to rescue her and then throw in the twist at the end that she knew all along what the plan was but fun was more important than escape. Same characters, with different plot and antagonist. That's much more entertaining than reading yet another "Luna celebration, fight the manitcore, oh no parasprites/poison joke while Twilight freaks about what Celestia will think but she likes what happened and it was 20% cooler in ten seconds flat" story.
Don't need OC ponies. But do need OC stories.