This question is partly inspired by this thread.
I am mentoring a student who claims: "I want to do pure mathematics because it is superior to any other subject in the world like applied math, statistics, science, and engineering". On one hand, I like it when he finds the thing he really loves. But on the other hand, I really don't want him to have such a belief that what he wants to do is the most important thing in academia while other subjects are inferior. Such arrogance will hurt him in the long run as a scholar. But I cannot give him a convincing reason since I also do research in pure math myself and have to confess that it was not until I found it hard to find academic jobs in pure math did I realize that pure math may not be so "superb" compared to other subjects.
However, I also do not want to tell him that he will find it hard to survive in academia if he does pure math. But if you believe I can say this to him you are welcome to explain in your comments and answers. I am happy to know!
It is not uncommon in the math community that people have the belief that what they do is sacred and pure math is the supreme of all subjects in the world. Even within pure math, I have seen the very hilarious hierarchy (this student is also aware of this without me telling him):
number theory, representation theory and algebraic geometry > topology and geometry > analysis and PDE
(although I cannot deny that Fields Medal and Annals of Math do favor people in number theory and algebraic geometry)
So, what can I tell him?