As a non-native speaker I encounter this phrase from time to time (in podcasts and such) and I’d like to understand the use (beside the literal meaning which is obvious).
Why would you say that? or sometimes Why do you say that?
To me, that sounds almost rude, like rebuking the questioner. However, the context usually leads me to conclude that this sentence is to be understood neutrally, in the sense of “I am interested in the background of the question”.
How should the sentence usually be used? Or does it depend solely on the tone the phrase is used?
Additionally, there are variations in tone of those and similar uses.
“Why would you say that?” = “What makes you say that?” = Please explain your reasoning.
Similarly, the above but in a sing-song or over-enunciated tone = “Now why would you say a thing like that?” = A coy or sardonic variation of the above, where the request for reasoning is rhetorical and unneeded because it is obvious.
“Why would you say that?” = I fundamentally disapprove of the content of your statement
Similar to but distinct from the above, “Why would you say that?” = While I don’t necessarily disagree with the content of your statement, I disapprove of speaking it aloud.