Every teacher wants a serious student. We don't want the most brilliant students, we want students who take classes seriously, who genuinely want to learn and expand their horizons. I have a hard time taking students seriously when they only want to take a class to get a credit, fulfill a requirement, or have fun.
Writing is also for serious people. Anyone who wants to get published needs to know the endless hours of honing the craft, revision, and head-slamming needed to get a manuscript out. Writing is not for the casual. I can understand the idea of exploring and testing the waters to see if it's right for you, but I have problems with the idea of "everyone has a novel in them" or the people who think wistfully, "I might write a book some day."
Writing is not something that just spontaneously occurs (though many a writer wishes it were so). It takes a lot of hard work. It takes dedication. Writing takes blood, sweat, and tears.
How many are willing to bleed for their writing?
Paper cuts can be nasty, though carpal tunnel is more likely.