I don't think that ¿Qué causas tristeza? is exactly what you want to express, Izanoni -- not only not to mean What causes you to be sad?, but even for the more general What causes sadness?

By happenstance, I am working throught the uses of the "artículos: un, el, or none (as represented by a 0 with a slash through it, the name for which symbol, I do not know)"

I have just completed an exercise on this very subject in Gramática básica del estudiante de español and think I have learned this:

You use "el" (i.e., los, las, etc., of course - in short, definido) when referring to Toda una clase de objetos o personas

You use the indefinido when referring to Cualquier objeto o persona de una clase

You use no article at all for una cantidad indeterminada de objetos o personas

So I think you must write - ¿Qué causa la tristeza?

Note that it is something else that causes the sadness...not YOU causing it...it is TO you that this sadness is caused:--) and so I changed your "causas" to "causa".

By the way, I hope that a more knowledgeable person can confirm this post....I, too, am only just learning these points of grammar.

And of course, if you wish to include "you" in the sentence, I think you would have to say ¿A ti que te hace sentirte triste?

What do you think?