I think valleyfair!'s comparison to movies is pretty apt. The first time you see a movie you end up liking, the experience won't generally get much better from there.. just as your first ride on a coaster is usually the most thrilling because you don't know quite what to expect. And with both movies and coasters, picking up some of the nuances with repeat experiences can make them very enjoyable. But as the coaster/movie gets more familiar and predictable, it starts to lose its glimmer.
Personal example: Toy Story is one of my favorite films. It made me feel so happy and excited as a child that I used to watch it over and over... to the point today where if you were to give me a random line somewhere in the movie, I could not only tell you who said it, but I could pick up the rest of the dialogue and play out the entire rest of the film from that point. Dead serious, I have seen Toy Story that many times. So yeah, I don't watch it much anymore because literally all of the novelty has worn off, but I guess because of nostalgia, the jokes that I now understand but didn't as a child, and the fact that it actually is a legitimately excellent film, I still enjoy it and consider it one of my favorite movies.
The same can be said of a few coasters in my count. Take Apollo's Chariot for example: my first rides on it made me feel just as excited and wonderful as my first couple viewings of Toy Story did (obviously in different ways). Over the years, I've come to ride it so many times that nothing about it surprises me anymore.. I know exactly where and how the G's are going to hit me, and that has dulled the experience noticeably. However, because of the way it made the feel on the first few rides, the fact that it's generally acknowledged as a high-ranking coaster, and the fact that I still enjoy it as much as I do despite having ridden it so many times, I still put it in my top 10. This despite the fact that other coasters have more recently thrilled me more (Talon, for instance).
TL;DR --- Re-riding a coaster definitely wears out the experience, but IMO in most cases it shouldn't deeply impact your positive perception of the ride.