A toddler who demands the same book for the fortieth time is doing something specific. The repetition is the mechanism rather than an obstacle to be worked around.
Prediction is the pleasure
On a first reading a child is processing everything at once. By the tenth, most of the effort is free, and attention can go to anticipating what comes next.
Knowing what is coming and being proved right is reliably satisfying to small children, which is why they signal the turning point before the page turns.
The book has become a small predictable system in a day full of unpredictable ones, and control over it is part of what makes it attractive.
Words are learned through stable context
A word encountered once in an unfamiliar sentence carries almost no information. The same word in the same sentence, alongside the same picture, is learnable.
Repetition holds every variable constant except the child's growing understanding, so each pass extracts something the previous one did not.
Storybook language also differs from conversation. It contains vocabulary and sentence structures that rarely appear in household speech, and repetition is how those become familiar.
The text becomes a script to join
Children begin completing phrases, then whole pages, and eventually recite the book while turning pages at the right moments. This is not reading and is a precursor to it.
It establishes that the marks on the page carry fixed content, that the words are always the same, and that the story moves in a particular direction across the page.
Those concepts are prerequisites for formal reading instruction and are acquired incidentally through favourites rather than taught.
Repetition is also doing routine work
The same book at the same point in the evening functions as a signal that the day is ending. The content matters less than the reliability of its position.
This is why substituting a different book at bedtime can produce resistance out of proportion to the change. The sequence has been disturbed, not the story.
Families who want variety generally do better by adding a second reading slot at another time of day than by replacing the established one.
How new books get in
Introducing a new book alongside the favourite rather than instead of it lowers the resistance, since nothing is being given up.
New books with a similar structure, rhythm or familiar character are accepted more readily, because much of the prediction advantage transfers.
Interest in any single title fades eventually and without intervention, usually once the book has been fully mastered and there is nothing left in it to anticipate.