I agree with above posters. It can't be explained. My wife had been to WDW before with her parents when she was younger. She didn't have much fun and neither did her parents.
Like a lot of people here, my family (god bless them) took yearly family trips to WDW. Over that time, you get better and better at planning, knowing your way around, etc.
Well, we took a family trip (My mother, father, brother and family, sister and family and my wife and daughter) in August of 03'. My wife was happy to go on vacation, but wasn't excited at all.
By the time the trip was over, she didn't want to leave. She couldn't believe how much of a difference there was on this and her past trip. She now tells everyone who is skeptical that you haven't experienced Disney unless you've done it "The Monroe" way.
As you know, the way is the energy, attitude, knowledge and passion you put into your trip. Most of us here on the boards have it. That's why we spend parts of our day contributing to this board.
Last year, my skeptical in-laws wanted to go with my wife and now two children. They were still skeptical from the last trip. They also had people telling them that they were going to be bored and wasting their money. They couldn't believe we were going to be there for 10 days!
After our trip my in-laws couldn't believe how much fun they had. Neither could the naysayers.
I'm convinced, the key to catching the WDW bug is related directly to 4 attributes: energy, attitude, knowledge and passion . Not everyone is fortunate enough to experience that.