the reason why 點解?

The reason why 可以用係formal writing English之上, 不過一定要加phrase.

e.g. 借用樓上朋友既例子, "The reason why she did it is still a mystery"入面 Subject係The reason, verb同object係 is still a mystery. "why she did it"係用黎補充The reason的.

外國樂隊The Click Five 有首歌叫做 "The Reason Why", 這在文法上其實是不對的, 但用於歌名上就沒有限制了.
參照該曲歌詞中 "That's the reason why when you're young you fall in love", 其實That's the reason 已經包含了句子的主謂賓, "why when you're young you fall in love" 係一句補充既phrase.
一般來說"why"是用來連接the reason同想補充的內容.

好少見人聽the click five 既歌OvO!!!

1. 理由,原因,動機[C][U][(+for)][+(that)][+why][+to-v]
The reason (that) she did it is still a mystery. 或 The reason why she did it is still a mystery.
她為什麼做那件事仍是一個謎。
We have every reason to refuse.
我們完全有理由拒絕。
We have reason to believe that he was lying.
我們有理由相信他是在撒謊。
The reason for her absence was that she was ill.
她之所以缺席是因為她病了。
2. 理性,理智;判斷力,推理[U]
3. 道理,情理;明智[U]
Why won't you listen to reason?
你為什麼不願聽從道理?
4. 正常心智,正常神志[U]
vi.
1. 推論,推理,思考
He is good at teaching his students to reason.
他擅於教導學生如何推理。
2. 勸說,規勸[(+with)]
vt.
1. 推論,推理[Y][+that]
I reasoned that he did not come because his old car had broken down again.
我推想他沒有來是因為他的老爺車又拋錨了。
2. 勸說[O][(+into/out of)]
I reasoned him out of selling his house.
我說服他不把房子賣掉。
3. 辯論,討論

ad.
1. (用作疑問副詞)為什麼
Why should you be so interested in my affairs?
你為何對我的事這麼感興趣?
2. (用做關係副詞)為什麼
These are the reasons why we do it.
這些就是我們為什麼要做這事的理由。
That's why I came here.
那就是我為什麼要來這裡的原因。
n.
1. 原因,理由[P1]
Robbie always asks the whys and wherefores when he is told to do something.
如果要羅比做事,他總愛問個究竟。
2. 難解的問題,謎[P1]
int.
1. (表示驚訝,不耐煩,贊成,異議,猶豫或引出另一種想法等)咦,哎呀,哦,當然,唔
Why, this is the very book I want.
嗨,這正是我想要的書。