Reading, aims and purposes
We read for the pleasure of it. At bottom, there's no other reason to do it. If reading is eating vegetables, and vegetables are an un-integrated side element of one's diet included only for nebulous reasons of promoting good health, then the enterprise is doomed. Many other pleasant and edifying pastimes exist, and there's no shame in preferring one of them.
Pleasure is not a sufficient reason to read. It can serve as a worthwhile guidepost in youth and in full maturity, but it can be deceptive. We read those books that do not come naturally to us, all the better to expand the range of future reading; there is no pleasure so delightful as realizing a previously foreclosed world has now opened. (I hestitate to say this because it can give off the wrong impression, but I go back to Zola and Anthony Powell and Márias because I find them to be relatively easy, which is not, I gather, the general feeling about any of them as writers. Difficult for me is Pynchon and the bleeding edge of experimental fiction, Thomas Bernhard and the like.) Reading also can claim the results ascribed to it: we read to learn, we read to see the ways in which other people are different, we read to see the ways in which we are the same. Those who write will read to pilfer tricks and techniques.
I have always been an avid reader, but I became a more sophisticated reader in grad school. There I learned the fine art of reading a great deal of text at high speed with good retention; none of these are unusual skills, all of them are learnable through practice. Once one begins reading at that clip, a number of things become evident: I can read for structure, read for style, read with a knowledge of context, debate the minutiae of word choice and sentence construction. I learned, very importantly, that there is such a thing as reading too fast. I read texts somewhere between the death of the Author and the author's intention, with the knowledge of what it means to write, and how the practical limitations of assembling a long text manifest. When I turned these skills towards fiction, it made a substantial difference in the books I chose and how I approached them.
Reading is, then, not about identifying themes or motifs, identifying symbolism, and all the other things generally taught in high school English curricula. A book--true of fiction, true in its own way of theoretical and philosophical reflections in a number of fields--is more than this. If a work of art is good, it exceeds any of its individual elements. To reduce it to its individual components--assuming one has a complete list of them, an unlikely possibility--is like thinking that a finished recipe is the same as its ingredients. When they are pulled out of their context within the work as a whole, something is lost. Reading only for thematic content is the way that people surround themselves with a chorus of voices that agree with their biases, and allows the reader to brush aside any moments of unpleasantness a novel might bring; worse yet, it encourages people to select books to read based on whether they are likely to be agreeable.
Re-reading, aims and purposes
In my old teaching statements, I would sometimes include a paragraph about reading a text four times: once for class, once in the act of having the reading discussed in class, once to produce a paper, once again for the final exam. The idea was that one reading is insufficient: one can get first impressions and some crude idea of what is going on. The second allows the reader to see something of the structure of the whole and how that structure might illuminate the various parts of the text. The third and fourth readings allow one to begin looking at particular recurrent elements in the text. One re-reads, in other words, because new things appear with each subsequent reading. (An old professor of mine once proposed a course centered around Machiavelli's Prince, which would consist of reading The Prince and then a famous book-length commentary, then reading The Prince again, then another commentary, with the idea that whatever the book meant would change radically from beginning to end of the semester.) We accept this as a given with songs, tv shows, albums, movies: periodic or traditional or obsessive re-consumption is a given. Novels are often made an exception for the tremendous time investment involved, but in an age of auto-streaming 13-to-24 episode seasons of prestige drama, the idea of limited time looks like an excuse. Re-reading shows the ways in which we have changed over time, always a useful corrective to our feeling of one continuous and unbroken reality of the self.