I don't know why.

我不知道为什么。

《柯林斯英汉双解大词典》

I don't know why everybody makes such a fuss about a few mosquitoes.

我不知道为什么所有人都为几只蚊子发这么大的牢骚。

《柯林斯英汉双解大词典》

I always end up having an argument with her, I don't know why, I just can't help it.

我总是和她意见不合，闹得不欢而散，我不知道为什么，我就是忍不住。

《牛津词典》

I don't know why I do it. I just do it."

VOA: special.2009.07.04

And I don't know why, but since time immemorial that is the way computer scientists have drawn trees.

叶子是在最下面的，我不知道为什么,但从记事以来他们就是这么画树的,这也许就是为什么我们。

麻省理工公开课 - 计算机科学及编程导论课程节选

But for me, I like to look at something and just say, "I like this. I don't really know why though."

但对我来说，我喜欢看着某个东西，然后说，“我喜欢这个。但我也不知道为什么。”

关于现代美术 - SpeakingMax英语口语达人

Don't know why I ever thought I needed, don't know why I thought I needed, don't know why I ever thought I needed you.

NPR: Dustbin Bands: The Surreal McCoys

That's why I don't actively seek reasons for drawing her, because I know I shall be pulling my punches.

WSJ: Her Maj: 60 Years of Unofficial Portraits of the Queen | Cartoon Museum | Too Gentle on the Queen? | By Richard Holledge

And I don't know why I do, except perhaps for what we'd named the dolls - toenail and fingernail.

NPR: Watching a Memory Shift over Time