Well, the first reason is to try something different. Another reason is Free-style cruising which I am very personally very excited about. I think it is a great idea to have dinner from 6:00 to 12:00 and let you come whenever you want to. Personally, I hate formal nights, and on free-style you can choose not to go to the areas of the ship where formal dress is required - that's a big difference. On my last Princess cruise I was a prisoner in my cabin all night because i didnt feel like donning my tux!
