(which I abandoned years ago, though I'm not sure why; must start it again sometime).
Sometimes it can be mood. I feel I sometimes have to put something away if I'm not feeling in the mood for it. For example, a few years ago, someone lent me The Terror by Dan Simmons. I had found it hard to get through and shelved it for a few months. Then I picked it back up and I didn't find it so hard to go through and enjoyed it quite a bit.