While it’s true that only the author knows what works in a story (until it gets published, anyway), getting early feedback from beta readers and editors can help improve a manuscript. This kind of feedback can expose blind spots, stress-test plots, and give an indication of how your book might be received by the broader readership.
The key is to read the feedback carefully, and not let it trigger an emotional reaction. When someone writes a negative review, it may feel personal and hurtful. After all, you’ve spent months or years slaving over your manuscript and now this person has said they don’t like it. But it’s important to remember that a good or bad review doesn’t owe you anything. All that matters is whether the reader got something out of the reading experience—enjoyment, entertainment, information, inspiration, or otherwise.
A negative review can point to issues that could be improved in a revision, or simply suggest a different way to approach the narrative. For example, if a beta reader points out that the precocious child in your novel is a trope, you might want to think about how the character will be developed in future installments.
The same can be said for a suggestion that you reduce your use of literary techniques that are too formulaic or clichéd. You might have to do some research to figure out what the reader is referring to, but if you look closely at their other reviews or their review history, you may be able to see a pattern.