My girlfriend is a great example of increased excitement with each trip. I took her for her first ever WDW trip for our 1 year dating anniversary, her college graduation and her birthday. We went NYE 08-09. She wasn't really all excited about going but did her best to humor me while I was trying to book and plan things. Everything changed the moment we stepped into MK and she saw Cindy's castle in person. We went back this past May and are booked to go this October. All she does now is talk about the trip, what she can't wait to see, can't wait to try, etc. etc. etc. We are doing a lot of things differently which is keeping the WDW experience fresh. She has become obsessed with our next trip. I'm kind of scared to see how she will be on future trips.
I think a lot has to be said about knowing what a WDW vacation is all about. I have talked with her about it and she has told me that while I was planning the first trip, she just thought it was going to be cute but kiddie. Now that she knows what it's like, she has a totally different view.