"du dig" .. you you?
I word-for-word translate it as "Why do you worry yourself about it?"
In Swedish "att oroa sig" - "to worry" is a reflexive verb.
Why för and not om?
can somebody explain why "why do you worry about THAT" is wrong? doesn't 'det' mean 'that' as well?
It's not. My answer of "Why are you worrying about that?" was accepted, so report it if it still isn't accepted.