Konnichiwa Mina-san!
to Fixelbrumpf-san!
"Why?"
"Naze?", "Doushite?", "Naze nanda?", "Doushite Nanda?", "Naze nano?", "doushite nano?".
Usually, the ending of word "-da" is used by male, and "-no" is used by female.
"Why does paper burn?"
"Naze Kami ha Moeru no?", "Doushite Kami ha Moeru no?".
"why do you always do this?"
"Naze itumo Konna koto wo Suru no?", "Doushite itumo Konna koto wo Suru no?".
to Elizabeth-san!
"Naze" is not Keigo, but "Naze" is more formal than "doushite". And Rquethe-san's teacher said that "naze" is more polite than "doushite", I think. I think "naze" is more polite than "doushite" too, but it doesn't mean "Keigo" or "Teineigo". It means that "Naze" is formal(or stiff) style and "Doushite" is familiar(or emotional) style.
NANGI