What makes Carnival special to me is more the "who" that we travel with. I have a teenage son who doesn't like to hang out with his mom too often nor has much in common with (turns out I don't play tackle football, ride an electric bike or obsess over teenage girls) so once a year (every Spring Break) a group of single mom's cruise with our teenage sons....so cruising gives us something to chat about, look forward to, enjoy together and make great core memories.
Now why specifically Carnival over other lines is the FUN, I have sailed other lines during Spring Break and the boats were boring by 9pm. Of course that can be situational, but myself and my teen just seem to have more fun on Carnival. Lastly, the on-board credit we get for being return passengers usually pays for my Cheers, my sons Bubbles and his WIFI, so that helps too.