We take about one cruise per year, and for me it's just about saving up. It isn't easy because we can only cruise during peak season, when fares & flights are quite costly. It helps that my son and I typically sail as a party of two. Once I invited my mother to cruise with us, and that really was noticeably more pricey, paying for 3 people rather than two.
Everyone has a different income & different types of expenses in their lives. Some people can afford it easily, others cut corners in certain areas to make room for cruising. And some people have access to better deals than others. People who can sail on Florida discounts & cruise offseason pay a lot less than those of us who are not from Florida & must cruise peak-season. People who live close enough to drive to the port can save a lot on travel expenses for each cruise.
To answer the OP's question, we've sailed on 5 cruises so far. We want to sail on the Fantasy one more time, as we didn't see Aladdin on our first Fantasy cruise. We also want to sail on each of the new ships when they come out. We'd never sail 50+
DCL cruises even if we could afford it, though. We like to do other things for vacation, as well.