Usage "reason for the" or "reason of the"?

Question

When you want to know why someone is requesting for something when writing an email, which should be used?

May I know the reason of/for the request?

Is there a difference between the two? Thanks.

Answer 1

The usual expression is reason for, if (as in the question) it's followed by the thing for which you are giving a reason. You might also use reason behind.

The first source to consult for this sort of question should be a good dictionary. Merriam-Webster's entry for reason has the example:

the reason for earthquakes

There are typically two different things that might follow reason: either identifying what is being explained, or identifying the reason. Consider that the reason for earthquakes is a shift in the earth's tectonic plates. As already mentioned for is the usual way to link "reason" to what is being explained

• The reason for earthquakes is a shift in the earth's tectonic plates.

Alternatively you can use why with a finite clause; Merriam-Webster's example is "the real reason why he wanted me to stay". Continuing the above:

• The reason why earthquakes occur is a shift in the earth's tectonic plates.

And behind, although it seems more common to use this when there are multiple reasons, as in Merriam-Webster's example "the reasons behind her client's action"

• One of the reasons behind earthquakes is a shift in the earth's tectonic plates; fracking and volcanic activity may also be involved.

If you want to name the reason then you might use of. Merriam-Webster gives "He was found not guilty by reason of insanity." This is common with "by reason of", but can sound a bit clunky in other contexts (usually you don't need to say "this is a reason", you just use a construct that indicates a reason, such as "because X"/"because of X"/"to X"/"for X"/"so that X"/etc).

Confusingly you can use "that" clauses in both ways. What is being explained (M-W as above):

That’s one reason that Stalmaster earned his honorary Oscar. (Los Angeles Times)

What the reason is (Merriam-Webster):

I don't want to go for the simple reason that I'm very tired.

Answer 2

X of Y expresses a relationship - X is part of Y; Y is a "container" and X is in it.

X for Y expresses a purpose/answers the question why? - Y is X's purpose/destiny.

May I know the reason of the request?

This sounds like the request is written on a form, and one of the lines on that form is "Reason: ______" and you want to know what that is.

May I know the reason for the request?

This is equivalent to saying "May I know why this request was made?"

Do both arrive at the same meaning? Yes, they are practically equivalent ... but the first one sounds much more formal as of implies the request was a "container" that possibly had several things that needed to be specified/known/committed before becoming an official request.