Anothing thing which has been mentioned in various ways (in addition to "comfort reading", which I will agree is an attractive factor) is that a good book grows with you, so that the more you learn, and even the more you read, the more dimensions that book takes on with subsequent rereadings. For those of us who enjoy intertextual resonances, there is also that aspect of the various texts altering the way each is read, so that reading those works again is by no means the same experience, but in many ways quite different. (The classic example I see so often is Homer's Odyssey and Joyce's Ulysses, where a knowledge of each has great impact on one's reading of either, making each quite a different book depending on whether you have read the other or not. And, to be frank, different translations of books can also have a wildly different impact. Try reading Homer in the original, then Pope's translation, then in the Andrew Lang translation, then in one of the numerous modern verse translations, and you'll see what I mean. The basic story remains the same, but the experience is anything but....)
In part, I suspect, it depends largely on how closely one reads, as someon who reads a book very closely, for more than just the plot or story, is likely to experience that book differently with each reading; whereas someone who reads it solely (or even primarily) for that, is much less likely to have that happen with them....