Does anyone else hate it when 2 characters in cartoons or anime get build up as a couple for them to only not become a couple in the end?

It really annoyed me when Dick and Barbara broke up on Batman TAS. I know a lot of people loved Korra and... I don't remember that girl's name... I really thought Korra and Mako made a really good couple. They did a lot of building that relationship up and I really felt like the carpet got ripped out from under us at the last second. TBH, I didn't feel like Korra's other relationship made sense. I also remember thinking how awkward it was when Lacus choose Kira over Athrun, but I actually really liked the Kira/Lacus relationship. Kira/Lacus seems like their personalities mesh better. I love Fred/Daphne but I wish they'd stop trying to make Velma/Shaggy a thing. I also think Adrien needs to hurry up and ask Marionette out. So, I guess it depends on the couple.

Yes, its highly annoying when we're let down at the end. However, some script writers like to mess with us.

Oh how I wanted to see additional seasons of Outbreak Company or Amagi Brilliant Park.

Most anime are produced for the sole purpose to promote it's source material whether it be a novel, manga or game series. A anime that ends with the story not yet resolved means it's meant to entice the watchers to read the original material.

Additional seasons only happen if the anime gets alot of DVD/Bluray sales.

No, but I like it when unlikely couples hook-up to share some taboo forbidden romantic love.

Any long running serialized work of fiction has this risk. The author usually set out to have two characters get together but then as he writes the story details changes his mind.

C'est la vie.

You get way too into that stuff my friend