I'm a fan of Disney cruising. I'm very big on the aesthetic aspect of the ships, enjoy the theming, love the classic ocean liner look, and find the staterooms to be spacious and pleasantly designed. When I've looked at photos of other lines' staterooms in particular, but also some of the common areas, I'm not as impressed.
I also like that DCL is family-oriented and pretty-much everything is G or PG rated. That just makes it more of an escape from real life.
And most of all, I like Castaway Cay and how peaceful, convenient and beautiful it is. I never sail non-Castaway tropical cruises on DCL, and wouldn't be interested in a tropical cruise on another line for that reason.
If another good line had a convenient sailing to some European ports I really wanted to see, I would do it. But that would be cruising for the ports & using the ship as a floating hotel/transportation source. When I cruise DCL, the ship itself is one of the destinations.