This actually is interesting in terms of information-seeking behavior, which is a thing librarians think about a lot and often actually study (some library jobs require you to publish, and academic librarians, for instance, will often use the students at the college they work at to study how they search for information in order to figure out how to best provide them services).
When you go for an MLS (Master’s of Library Science, which is a thing, and which is usually required for “professional-level” library work [which is also a weird and contentious concept that I won’t go into here]), one of the things you study is the organization of information. This deals with how to determine what a book or other material is “about"—a concept we tongue-in-cheek call “aboutness"—and how to convey that to a potential user of the item and make it easy for them to find. Things like keywords and subject headings, do I put this book about how often wild birds attack aerial drones in with books about birds or with books about technology, if its a fictional novel do I put fantasy in it’s own section or mix it in with all of the other fiction, so on and so on.
OP is organizing books by how they would look for them. OP’s partner is thinking in terms of aboutness. This is a system that works for OP because it’s their personal library: they know basically what books they own and they only own books that are relevant to them, and if they know what the book looks like, that can be a quick way to find it.
In a library that assumes the public (or people who do not own that particular collection of books) are using the collection, that doesn’t work. Books are often re-issued in multiple covers, or re-bound in new covers when they get worn out, and if the user doesn’t know what the book looks like or is expecting a different cover, they’re lost. That’s why non-personal libraries used standardized cataloging systems like the Dewey Decimal System or Library of Congress System to organize a book by what it’s “about”, and then put books about the same or similar topics together, marked with labels and signage so a person unfamiliar with the book or collection can find their way to it.
Basically, OP’s system works for their own personal library, because it’s best suited to how the primary user—OP themselves—looks for books. OP’s librarian partner is coming from a background of thinking in terms of a public-facing collection, where aboutness is the key criteria and communicating it to a user unfamiliar with the collection is the priority.
And also, OP is a monster.
