I'm in my reading groove again (more books read this month than in the previous four months combined) and I think it bears mentioning that the act of reading is called an active act because you can't do it haphazardly, on "cruise control" (as we might other activities, like walking or driving). Well, I do that sometimes--either I don't get the story or the novel is really full of padding that it doesn't matter I gloss over that information.
Anyway, reading takes concentration and focus. People I think get immersed in a book when they need little effort to maintain either. Of course for me, there are several distractions that can get in the way of immersing myself. It can be something as simple as noise. Or something as biological as hunger. And then there's the point when you get sleepy for one reason or another. Personally, when that happens, I drop the book and grab a power nap.
What I find interesting is how concentration and focus affects our brains. I mean people talk of being "in the zone" when participating in a game, playing sports, or some such activity. The same applies for me when reading--when you're reading at quadruple your reading speed, when everything just "clicks" and starts making sense, when you understand every single word and expression.