It doesn't seem all that strange when we watch a favorite movie so many times we can quote the dialogue word-for-word, or when we listen to the same song or album repeatedly. But with books it is a bit different. Watching films and listening to music are essentially passive forms of entertainment; with a book there is a serious commitment of time, and the inability to do anything else while reading. You can use a power saw or fry chicken and listen to Beatles at the same time; try doing those things while reading A Farewell to Arms and you may end up in the hospital.
I believe there are several reasons we go back to the same books over the years. For one, we know what we're getting. There is nothing worse than spending hours or days reading a book only to find out it wasn't really worth the time, especially if the ending was a disappointment. With a book you've read and loved, you know you won't be disappointed.
Another thing about rereading a well-loved book is that in many ways it is like visiting an old friend. I find that the books I tend to read more than once have especially strong and well-written characters in addition to a good story. Spending time with these characters again is like running into an old college roommate. You may not want want to live with them again, but it's enjoyable spending a few hours together.
Finally, the books I reread almost always give me something new each time I read them. Whether it's a passage I somehow didn't catch before, or an event that speaks to me in a new way, there's always something fresh about them. For example, I have read The Razor's Edge every year for 30 years, and each time I get something new out of it. The book hasn't changed, but my life has, and the novel reaches me in ways in my 40s that it never could have at 20.
So while I encourage everyone to read as many books as they possibly can (it's the key to a well-rounded life), be sure to take the time to go back and read the books you loved again. They'll wait for you and welcome you home every time.