I still don't quite understand, why some couples in fiction look cute to me, but others look mawkish and cause annoyance. And the previous answers haven't clarify the picture for me, despite there were several interesting thoughts.
I can definitely name only one aspect, which make relationships look insincere for me: overwhelmly demonstrative "romantic" behavior. I mean couples, who are overdoing "romantic" things at every opportunity and say how they love each other every five minutes, like they don't know how else to demonstrate the reader, that they are in LOVE, LOVE, LOVE.
For example, I was really irritated by Marco and Ai - couple, who share the Exchange Diary - in Mirai Nikki. I've skipped a couple of series because of them.
Sorry for necromancy, I'm just really curious to hear more opinions on it, cause the thematic is of current interest.