Tropes. We know they are common character types, devices, situations, etc. that crop up repeatedly in many books or movies. Some people love certain tropes and hate certain ones. Why?
Been there, read that: The common answer is it's not new or fresh. Some people dislike when tropes give the story a contrived or predictable feel. But is that really the reason a reader didn't enjoy the trope? Was there something in its delivery that didn't give a "wow" factor? Was it too similar to another book that might've done it better in the reader's opinion? This is why I like to set up predictable tropes and then disrupt them to something unexpected. Take the love triangles in romance. I love to lead the reader to believe in this common book situation, that someone is torn between two people. Instead, I make it clear the character loves one and not the other, shifting it to the unrequited love trope.
Believability is relatability: Sometimes, as readers, we cannot fathom certain tropes because we have not lived them. I never was in love with two people at once, so a love triangle doesn't seem common in the real world to me. I'm sure others have, but to me, the relatability is missing, so I don't love that trope (don't hate it either). Then again, other tropes I have lived and completely relate to. I'll spare the details of my love life, thank you very much. The point is how we as readers relate to the trope can gauge how believable it is to us; unfortunately, if we find things far-fetched, we might see them as contrived. This is not to urge authors away from tropes. Many people enjoy the love triangle trope; being adored by two hunky men is definitely some people's cup of tea.
In short, tropes are things that help us enjoy certain genres. Be sure to research the implicit trope rules. Like murder mysteries ending with the reveal of who did it, romance should have a happy ending. You can bend or twist certain ones, but research and read your genre a lot to see what these canonized tropes are. It'll lead to better sales, reviews, and readers that will come back for more.