This is a question that I think about often. After 15 cruises and one coming up in late October, I believe that the excitement does not lessen because I know that each cruise is different and will bring new experiences. Just when you think that you have seen it all there is something special for you to remember. On our last cruise we had an especially enjoyable water excursion out of Puerto Vallarta that included snorkling, kayaking, whale watching, walks on a truly lovely beach, unlimited food and drinks (and one of the most beautiful days that I have ever experienced in my 62 years).
Hope that this answers your question.