I ask because I don't.
Once, back when I was twelve or so, I built a little cave behind some cupboards and some junk in the spare room, lined it with some cushions and a motheaten quilt and hid away in there every afternoon after school, reading myself into oblivion for hours at a time. I went through books like air back then. I could read a whole book in one sitting, ignoring any kind of siblings-and-pets chaos that was erupting around me; my parents used to take me shopping, leave me in the library, and find me at the end of the day, usually carrying a pile of books I could just about see over the top of. I read a lot now, but nothing approaches the kind of total, overwhelming, put-that-book-down-and-look-at-the-scenery! immersion I could manage back then.
Other things haven't changed. I still read four or five books at once, and I still have a pathological aversion to stopping at the end of a chapter. Books I like get re-read over and over again (although not quite in the way I used to when I was younger - read the last page, turn straight back to the first page, and carry on without a break). It's not quite the same, though.
This is what I'm reading at the moment:
Ruth Brandon, Other People's Daughters. The lives of governesses in the eighteenth and nineteenth centuries. Like Brandon's other books (at least, the ones I've read), it's very good and very detailed at the same time as letting you know exactly who she disapproves of.
Danny Wallace, Yes Man. It's very good. I've liked Danny Wallace since Are You Dave Gorman?. This one, though, I'm reading a couple of pages of before I'm going to sleep, sometimes, which is why I've been reading it since January and am only 2/3 of the way through.
Virginia Nicholson, Singled Out. The lives of single women after the First World War, and fascinating. I can't say enough good things about this book; in fact, I'm deliberately reading it more slowly now so that I don't have to finish it.
Markus Zusak, The Book Thief. Lent by someone who thought I'd like it. I've only read the first two dozen pages or so, of about four hundred, but, yes, it seems good so far.
China Mieville, The Scar. I bought this in October and only started reading it this week, tsch. As with his Perdido Street Station, which is set in the same world, I'm very impressed at the same time as being vaguely annoyed by how the biology works - how is it a human with the head of an insect, or the body of a crayfish? How? I know, I know, it's fantasy, and he doesn't care about that sort of realism, but dammit, I care! Which is why I'm reading this one slowly, too.
Hmmm. Needs more short stories.
I read differently than I did when I was younger certainly. I think more closely and attentively, perhaps more obsessively; and also readier for the book not to be what I am expecting it to be. (Though rigidity on this point continues to be an issue for me.) I mostly read one book at a time these days though I may have a set of rereads in the works at the same time. Right now I am reading (mostly) Seven-Tenths by James Hamilton-Paterson, and re-reading (mostly) Borges.
I read constantly as a child, devouring up to six 'adult' books a day and fortunately lived opposite the local library. But I read for pleasure.
Now I also read constantly, pretty much from morning to night every day, for my job. And, this can be exhausting. As a result, I rarely read for pleasure except on holiday. And I do find this frustrating.
I read obsessively as a child. It was a large part of my identity. My family was very religious and consequently, we weren't allowed to do almost anything on Sundays. I would come home from church at noon and read from noon to midnight. Anything and everything I could get my hands on. It was always a joy.
These days I find, to my horror, that I read less for pure pleasure than I used to. Newer and crappier reasons I am reading books include:
1) To enhance my ability to understand and interpret popular culture (meaning that I read things I don't find all that great, just so I have some modicum of understanding as to what other people are referring to. Bolano is a good example of this)
2) To improve myself. Not self-improvement books, but books that I believe will help me be a better person. For example, Dostoevsky's writing style makes me feel like I am being suffocated by a feather pillow, but I read 3 of his books last year because I felt I "should."
I also read for my job (although editing environmental impact studies can hardly be called "reading") and I definitely agree that it takes away from a lot of the mental energy one has for other types of reading.