Question about tag question

I'm curious, why do you think Mary chose "is it" as a tag question, instead of "isn't it"?
When do you use "is it" as a tag question in a declarative (opposite of "negative sentence") sentence?

Hmm.. That's a very good question. ☝
Using "isn't it" in that situation wouldn't give the same meaning to me. It would be like she isn't sure and is asking him if the shoe is on the other foot. I'm finding it hard to find a way to explain what meaning "is it" has here, so I look forward to hearing what other members have to say about it.

To me, Mary is implying that John had gotten angry at his room mate in the past and now he is in the same situation. So Mary is just asking him how it feels, knowng that he has done the same thing to someone else. She isn't really asking for an answer.

That style, nekocat, is a rather old style. You'd almost never here it in American colloquial English, probably seldom in 'Down Under' (including NZ) colloquial English, and nowadays in UK or Irish English, you'd surely hear it much less than one may have even 60 years ago.

I take that as being an ellipsis for is it not. Nekocat san, have you noticed that style being used often in conversational English?

Refer back to this thread for additional info nekocat:

I wonder if ...not

In this context my immediate perception of 'is it' versus 'isn't it' is that 'is it' declares the shoe to be on the other foot with the interrogative aspect being lipservice. In some cases 'isn't it' is more like a 'sodesune' which is possibly asking for confirmation or validation. For example, imagine you're going to Osaka tomorrow:

nekocat: I'm going to visit my girlfriend tomorrow
bk: Ah, going to Osaka are you?

now.. using the negative

nekocat: I'm going away for a bit, see you soon
bk: Hmmm, you're going to Osaka aren't you?

The 'aren't you' here could have a smiliar meaning to the 'are you', which indicates foreknowledge, but, it can also be asking for validation or confirmation.

Your original example is one of those cases where it goes both ways.