I think it's a pushing of the emo-buttons to elicit gut responses from the readers. Not so much decent plotting or setting or attantion paid to all the other aspects of storytelling, as it is a lens on the emotions of the pairing involved. For instance, if I'm writing Draco POV and it concentrates on his feelings for Harry to the exclusion of all else (that seems to result in UST/yearning/atmosphere of unrequited for me), then there's an element of angst involved, as (again, personally) I think people are ultimately alone in their heads and one never knows or is certain of just how much or how deeply the object of one's affections reciprocates. But I love happy endings, so...the angst usually resolves to that, at the close.