2. Your idea isn’t compelling (though your writing may be superb).
3. The agent has no idea how to sell this book, even if it is well written and the storytelling works.
4. The agent is clearing the desk before or after holidays and didn't read your query at all (It happens sometimes, I suspect).
5. The agent just sold something similar.
6. You misspelled the agent’s name (it can look sloppy) and/or have other typos in your query.
7. Your query is not compelling enough, even though your book is great.
8. You have invented a new genre of novel and no one can grok it. (Which may or may not mean it’s unsellable.)
9. You have a personally offensive facebook, twitter or other social media feed. (Yes, this can be a black mark against you.)
10. You have racist or sexist stereotypes in your book.
11. The last book of the type you are trying to publish was printed forty years ago, and times have changed.
12. The YA Mafia has blacklisted you. (JK, this isn’t really a thing.)
Notice that some of these mean that the solution is to stop querying, work on your writing, and then resubmit. To some of these, the solution is to fix the query and send it out again. And to others, the solution is, keep sending it out but to a wider group of agents because the ones you’ve hit so far aren’t the right ones to represent you.
I would say it’s also always the right solution to keep writing. Whether or not this is a good book that’s being rejected, you’re a better writer than you were with this book now. So keep writing, keep submitting, and keep looking to the next book which will always ALWAYS be better because you’re learning and growing as a writer who reads other stories, right?