I often abandon books if I'm not in the mood for them, or I'm not liking them. Usually I then get rid of the books by selling them on. Recently though I have been giving a lot of books a second go, usually ones that everybody seems to have loved, and by and large I've found that I have enjoyed them all the second time round.
Examples are: The Time Traveler's Wife, Small Island, Julie and Julia, The Lovely Bones, The Woman in Black, Raven Black - I could go on.
I think it's simply a case of changing tastes.