What is the opposite of "why not?"

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?

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:

• Why not go there in person?
• Why not eat at home?

When we make a reply to a suggestion like this, just repeating a positive form of the question will be ineffectual:

• A: Why not go there in person? B: Why (go there in person)?
• A: Why not eat at home? B: Why (eat at home)?

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:

• A: Why go there in person? B: Why not (go there in person)?
• A: Why eat at home? B: Why not (eat at home)?

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:

• You can't swim!
• I can (swim)!

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:

• Why be an ass about it?

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:

• *Why do go there in person? (ungrammatical)

We need a different auxiliary if we want to introduce some positive emphasis. The modal verbs WOULD or SHOULD are good candidates here:

• A: Why not go there in person? B: Why would I?
• A: Why not eat at home? B: Why should I?

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:

• Why Tom in the first place?

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:

• There was not nobody there.

... 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:

• Why not invite Bob?
• Why not not invite Bob!

It is simply Why? in your case.

Simply- why. You can add why this /why that/why, yes/ why, no according to the context.

Technically, 'not' is used to indicate the absence of something. Eg. She is not here. / This is not what I ordered.

This indicates the opposite of not shall describe the presence of the thing that was accompanied by not. Same goes for when you use it with why.