We have very similar cruising tastes! Carnival's product is a perfect fit for me. I don't want to dress up; I don't want five-star, white-glove dinner service; I don't care if the ships are aging or décor is dated; I like tons of trivia and silly deck parties and bargain cruises. I haven't even sailed Carnival's bigger/fancier ships; other than one cruise on Splendor and one on Freedom I've sailed nothing but Spirit class and Fantasy class for my 29 Carnival cruises. Put me in my beloved bowling alley balcony on a Spirit class ship and I'm a happy gal.
I tried RCL once and Disney once early in my cruising career and have stuck with Carnival exclusively for the past 12 years or so. The only two things that would get me on another line would be an itinerary I *really* want to do that I can't get on Carnival, or if somebody else is paying (I'll be doing NCL to Alaska next summer on someone else's dime, but I'd never book that myself and it doesn't appeal to me any more than Carnival).