"Sir, you are too negative."
Translation:Meneer, u bent te negatief.
"Meneer" is quite formal so that would cause an inconsistency with the informal "je".
I agree, and am happy that duo forces us to use u, but afaik grammatically je should be correct (please do correct me on this). and we tend to use it in my native language as well (it is I guess playful but not wrong)