When someone says "why not [something]?" I often want to reply with "why [that thing]?" However, if they don't actually state the "[something]" and just say "why not?" what is the correct opposite question? Is it "why yes?" or "why to?" (for an action) or something else?
Learn English – the opposite of “why not?”
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Best Answer
Why not [verb phrase]?
The problem that the Original Poster faces is that this question, Why not X?, does not have an auxiliary verb. More often than not, [X] in this situation will be a verb phrase:
When we make a reply to a suggestion like this, just repeating a positive form of the question will be ineffectual:
The reason for this is that this response needs contrastive stress. The positive polarity of the response needs to contrast with the negative polarity of the original question. When we want to use contrastive stress in a negative sentence, this is easy, we just stress the negative word not. Notice that if we reverse the order of the two questions, there is no problem with the negative response as a reply:
However, when we want to add contrastive stress for positive emphasis, we need to stress the auxiliary verb. Here is an example with the auxiliary verb can:
In the Original Poster's situation there is no auxiliary verb in the original question. This is because this why not construction uses a plain form of the verb, which doesn't use an auxiliary. We can see that it's plain form and not present tense by using the verb BE:
Here we see the plain form of the verb BE, not a tensed form such as are or is. This question is non-finite, and non-finite verb forms do not allow DO support, so we can't just insert the dummy auxiliary, DO:
We need a different auxiliary if we want to introduce some positive emphasis. The modal verbs WOULD or SHOULD are good candidates here:
Would in the first example presents the going there as hypothetical or counterfactual. Should, in the second, addresses the idea that the speaker thinks that going there is a good idea. It addresses the implied deontic modality of the original question. The fact that these versions have auxiliaries, allows the speaker to stress the auxiliary for contrastive effect.
Why not [Noun]?
If the original question involves just a noun, then we have the same problem. It is difficult to stress lexical words (normal nouns, verbs, adjectives) for positive contrastive emphasis. So in response to Why not Tom?, Why Tom will only work if we put a huge stress on why. It might be better to add another phrase on to the end of the question to bring out the contrast:
Another solution
We have said above that it is straightforward to add negative contrastive emphasis to questions such as these. We just need to stress the negative word not. Now, it is a peculiarity of standard English that it does not like double negation (or so it is said). What's meant by this is that double negation in standard English is often understood to cancel itself out. So if we see a sentence such as:
... in standard English, we might understand it as meaning that there was somebody there. Now the fact that double negation cancels itself out like this gives the Original Poster another option. They can use a second not in the question to provide the contrastive stress, but the sentence will still have a positive meaning. This could give the sentence a slightly ironic or sarcastic undertone, but it can also be used with comic effect: