Carnival Cruise Lines caters to a younger crowd. Just look at thier advertising, "The Fun Cruise". Look at the tv spots. Lots of younger people partying. Lets face it, the college age crowd tend to be a bit louder, a bit bolder and a bit crazier.
That's not a bad thing or meant as a slight. We were all young at one time. Now, I at least, am older, more mature, with children (one in college) and I'm looking for something a bit more sedate.
People having sex on the balcony may be okay when one is 25, however, I really can't see the wife and I doing the deed while hanging over a railing on a cruise ship! We just aren't that flexible anymore.
Also, here is another fact. CCL tends to have many cruises that are less expensive than other cruise lines. To be fair, they also have some cruises that are comparable in price to DCL, RCCL or Hal and the like, but most are significantly cheaper.
What does that impy? Well, the younger, less affluent are going to tend to flock to Carnival. Again, this is not to say these people are evil or something. It's just the younger crowd tends to be rowdier.