Roland Barthes explains that rereading is difficult because when you reread, you are performing a deeply counter-cultural act. He writes that « the commercial imperatives of our society, oblige us to squander the book, to throw it away on the pretext that it is deflowered, so that we may buy another. »

Barthes writes that « rereading is no longer consumption, but play », and that probably says it best.

The new book you’re looking for is probably already on the shelf – right where you left the old one. « Rereading, an operation contrary to the commercial and ideological habits of our society, which would have us “throw away” the story once it has been consumed (“devoured”), so that we can then move on to another story, buy another book, and which is tolerated only in certain marginal categories of readers (children, old people, and professors), rereading is here suggested at the outset, for it alone saves the text from repetition (those who fail to reread are obliged to read the same story everywhere), multiplies it in its variety and its plurality: rereading draws the text out of its internal chronology (“this happens before or after that”) and recaptures a mythic time (without before or after); it contests the claim which would have us believe that the first reading is a primary, naïve, phenomenal reading which we will only, afterwards, have to “explicate,” to intellectualize (as if there were a beginning of reading, as if everything were not already read: there is no first reading, even if the text is concerned to give us that illusion by several operations of suspense, artifices more spectacular than persuasive); rereading is no longer consumption, but play (that play which is the return of the different) ».
— Roland Barthes, SZ : An Essay, Translated by Richard Miller, Hill and Wang 1974

With few exceptions, I don’t reread entire books. I do refer to complex and favourite passages. My justification is that there is so much I haven’t even touched, so is it better to master once-experienced material over the expansion of material? In defence of rereading, sometimes perspective has changed and additional experiences provide different facets for reflection—and this has happened to me—, but how does one determine the value of retrod paths?