We also first "discovered" SXM on cruises. We have taken over 50 cruises and very much enjoy them - of course, we also enjoy land vacations - come to think of it, we enjoy anything that gets us out and about.
We started cruising with young daughters - taking a week cruise every year. We think that for a family vacation, cruises are great. Everybody can do their own thing - separately as desired during the day - but then come together for dinner and the evening. No endless discussions about where to go for dinner - maybe your family can avoid those, we seem to have trouble when there are lots of choices. And, those dang vegetarians who want every night to be Thai ....
We now have a family group of about 8 that cruises over the Christmas/New Year's holiday every year - again, we find that the structure is not a negative, but a plus for our group.
If it is just the two of us now, we do tend to think about a land vacation as opposed to a cruise. But, we have cruised enough that we know, and consider friends, some of crew on certain ships - and, when you cruise a lot you also get to know some regular repeat cruisers. I doubt that we ever will be people who stay on a cruise ship for months at a time, but for a relatively few people it is what they really enjoy and how they want to live a good part of their lives.
I think a significant part of it is simply how you feel about being on a ship out in the water - for whatever reason we find it relaxing and enjoyable just to be on a boat/ship. I do agree that this kind of feeling is somewhat diluted on the big ships of today - but as long as you can sit and look out over the water to the horizon, it still is pretty great.