You’re not going to like every book you pick up in a bookstore. There are a lot of books there, covering a lot of topics, and you’ll first go to the section(s) that have the kind(s) of books you like . . . In fact, you may never go to any other part of the store. And that’s okay.
Even when narrowing it down to the fantasy or biography or whatnot, you won’t like all the books that are there. You might like a lot of them. But you’re going to put back most of what you pick up. (Well, I’m making an assumption there; you may very well stack up a lot of books, but let’s be honest: Will you ever read them all? Probably not.) You’re going to look at the covers, the titles. You’re going to flip the book over and skim the back, or else you’ll open and read the flap. You’ll glance over the first page or two, then skip ahead to a random page. You’re looking to see if the writing “sounds” right to you, whether the book catches you from the start or if it seems like a slog. You’ll use all this data to decide whether to buy the book or not.
Of course, sometimes it’s something you’ve heard about somewhere, or it’s an author whose name you recognize or whose other work you’ve enjoyed.
But I’m talking random books by people you’ve never heard of. There are thousands of them, and you have to narrow it down to the handful you can reasonably afford to buy and (hopefully) have time to read.
This is what agents do, too.
They are in a virtual bookstore of as-yet-unpublished books. They are skimming the back covers and book flaps (known to them as synopses). They are testing the first few pages to see if these manuscripts are the kinds of books they want to read—more than that, are they the kinds of books the agent wants to work on and with for months and years?
Like anyone in this world, an agent won’t love every book (query, manuscript) he or she receives. They, like you in a bookstore, are just looking for a few to take home. And just because one agent or reader doesn’t love your book, that doesn’t mean someone else won’t. The agent puts your book back on the shelf for someone else to come along and discover.