On the courtesy use of “Konjunktiv” and its origin. Is it reciprocal?

Question

There’s a use of Konjunktiv meaning politeness:

Ich hätte gerne (…)

First, I wish to know why does this form denote Höflichkeit. (My only guess – influenced by my native language – is that this form is used as a the tacit preamble in brackets:

[Wenn Sie mir das geben wollen,] hätte ich gerne (…)

instead of giving an order.)

Secondly, if this “being kind” is reciprocal, that is, if the person who is offering a service says e.g.

Hätten Sie gerne (…) ?

which effect does it have? The same politeness or is actually an undecisive way of offering something?

Answer 1

EDIT: @Em1 raised some good points in the comments below. My answer is very specific to the given example and similar constructs, and I’m not sure it holds much water in a more general way, so take it with a grain of salt. Below is my original answer:

Emanuel is correct, the Konjunktiv here (“Ich hätte gerne …”) is, strictly speaking, only expressing a wish (literally translated: “I would like to have …”).

I say ‘strictly speaking’ because it is usually used to make an order, like “Ich hätte gerne ein Bier”, but it is much more polite than using an imperative (“Gib mir ein Bier!”) because you don’t order people around — at least not literally. While there is an implied expectation that the barkeeper will fulfill my wish by giving me a beer, in the words themselves there is no order or anything besides the statement that, you know, a beer would be really great to have right now.

The other side is basically just the question version of this, as a polite way of offering something.

Answer 2

On Google Books, I found a 2010 book on Mood in the Languages of Europe consisting of scholarly articles by various experts. The two German Konjunktiv forms, like the English subjunctives, are examples of irrealis moods. Here is a relevant quotation from the article on Mood in Latvian and Lithuanian:

Requests and expressions of opinion are frequently in the irrealis form for reasons of politeness. […] Yet the suggestion of politeness may be achieved not only by representing the act of volition, opinion etc. as unreal, but also by depicting the object of an act of volition as unreal, or just possible.

Here is another from an article on Mood in Modern Georgian:

The optative [a form of irrealis mood] can be explained in the vein of politeness theory, where indirectness is a form of (negative) politeness: while the imperative is a “direct” expression of volition, the subjunctive does not necessarily require the speaker to be the addresser of an imposing request.

Another from a paper on Mood in Czech and Slovak:

The hypothetical, “non-impositive” semantics of these main clauses is loosely associated with politeness in Czech and Slovak; thus, a 1st person conditional declarative may be used as a polite offer, and a 2nd person conditional interrogative as a polite request.

One from Mood in Swedish:

A further, cross-linguistically well attested use of the preterite subjunctive is to indicate politeness.

Mood in Dutch:

The indicative counterpart of (19) [“this background may serve as introduction”], given in (19') [“this background serves as introduction”], is merely a factive statement and does not suggest any of the ‘hope of approval’ that was expressed by the speaker in the original utterance.
[…]
Likewise, the speaker in (17) and (18) would definitely be less polite if he had used an indicative rather than a subjunctive form without applying some alternative politeness strategy.

Mood in Estonian:

The pragmatic use of the condition[al] to express politeness is widespread; it often occurs together with a modal verb […]

I think it’s pretty clear from these quotations what the linguistic position on the phenomenon is: It’s very widespread (though in some languages such as Greek or Russian it doesn’t exist at all or is quite different), and the use of irrealis moods for politeness tends to be related quite logically to other typical uses.