For some, it is fashion. The reason much fewer people take it up than vaping is that the world still works. However, for some it is not fashion but a decision. A decision that in most cases in my opinion is the wrong decision.
Before you answer "Then if you feel stressed over being a man, don't transition!" is not a real answer either; these are teenagers we're talking about and they will make foolish decisions. There are better ways to treat gender dysphoria than what is practically cosmetic changes to look like the other sex. As I said, only few, borderline cases should result in surgery.