When I was in high school, there was a sudden astrology epidemic among teenagers. Symptoms included asking people what their zodiac sign was, then giving them knowing glances. Every other young person had a copy of Linda Goodman's Sun Signs, but they were amateurs.
Those who had levelled up would have a copy of Love Signs, a copy the size of Infinite Jest or possibly The Lord of the Rings, if it was ever published in a legible font size. They'd look up the potential relationship they could have with a Capricorn or a Cancer or whatever, before deciding if they should like you. I know people who still do this, while laughing at their parents for wanting to match kundlis. (These are the same people who have to look up 'irony' in the dictionary.)
With those books, we learned to put ourselves in boxes. Oh, he's not a flirt, he's just a Sagittarius. I'm a neat freak and overtly critical of, well, everything because I'm a Virgo. Not because I'm an asshole or anything, honest.
Then, a few years later, there came Harry Potter and kids started putting themselves in other boxes. I'm a Gryffindor, I never back down from a dare. Yeah, well, as a Ravenclaw, I assure you that's a fucking stupid thing to do. Meanwhile the Hufflepuff is quietly chomping popcorn in the corner wondering why we can't all just get along.
And then, before the poor Slytherins even had a chance to get over being called calculating bastards, we got Myers-Briggs. And a whole new generation had a brand new way of classifying themselves and their peers. Oh, her? Stay away from her, she's an ENTJ - she'll probably judge your shoes sooner than look at you. Just stick with us, we're all INFPs. (Like that's any consolation.)
This is sounding like a rant now and I can sense you're confused. So what, you ask. There's no harm in all this. These are all just fun things to do as a young adult.
I think this shit is dangerous at that age. Because when you're a young adult, you are trying to find your identity. And that's not easy because it doesn't exist yet. Not properly. As a teenager, your job is to fill yourself with things you like, things you hate, bits of people you love, people you despise (not literally, that would be serial killer talk), try stupid shit, succeed, fail and try again. Because that's what it takes to discover who you are. At that age, the purpose of your life is learning what kind of fully grown adult you propose to be. And at that stage, you shouldn't have to choose between Brave, Clever, Nice or Evil. The hell kind of choice is that?
See, these seemingly harmless ways of arranging ourselves into categories hinder character development, man. If I know I have certain qualities that are terrible, knowing that I have them because I'm a certain type of person gives me a very handy excuse to not to anything about it. (See Virgo, above.) Seriously, what motivation do I have to stop running up credit card bills buying smoked brie at Food Hall, if I can say, "I can't help it! I just love luxurious things, like all Tauruses do." It's easy to succumb to that logic. I should know, I have spent several years being insufferable because I thought that's who I was ordained to be by my zodiac sign. And guess what? Nobody comes and tells you that how you are, doesn't dictate how you can be. They just call you an unprintable word and move on with their lives.
But now imagine this: there are no categories. Just genus Homo Sapiens. Hmm? Period. Now, you have to work at carving yourself into the kind of person you want to be. You have to become both the sculptor and the block of marble. You can choose to be brave and ambitious but lazy. You can be an intelligent coward whose goal is world domination. You can be analytical but still have empathy. You can be an introvert and friendly at the same time. You can be born in September and not expect your date to have perfect grammar. Or at least not annihilate the poor things if they don't. These are all reasonable possibilities that teens should be free to explore. And they won't be that, if they put themselves in boxes.
Because fun as astrology or Pottermore or pop-psychology is, they all have the potential to become prescriptive, rather than descriptive. And do you really want to mold your personality according to what some author thinks? We're horrible, we kill characters to make you turn the page!
So in conclusion: stop relying on other people's grouping systems and build yourself from the ground up. Now excuse me, while I go look at my July horoscope. It's okay, I can, I already know what I am: incorrigible.