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Let's say I'm asking a question here on Stack Exchange, and finish the
question by saying "Why is this?"

Is なんでこれ the most common way of saying this? If not, what's the best
alternative?
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2018-06-02T08:21:09.863
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2018-06-03T01:41:48.877
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question
word-choice
Is なんでこれ the most common way of saying "why is this?"
Direct translations are usually not a good idea, and in general it's best not
to ask "how do I say this in Japanese?" but "what is the natural thing to say
in Japanese that is closest to what I want?".

`なんでこれ` is definitely not what you want; it's difficult for me to articulate,
but it sounds like you're asking "why this specific thing among other possible
things that could have been chosen?". Somewhat like English's "why this?" (as
opposed to that).

The most common structure I see for this kind of question is simply `なんで？` or
`なんでですか？`, in something like

> アメリカと日本の平均体重は全く違います。なんでですか。

If you really want to emphasize `why is this`, you can also explicitly say
`これはなんでですか。`
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2018-06-02T16:00:36.460
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2018-06-02T16:00:36.460
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answer