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Many languages use the plural as respected mood for a singular (even English use "you" which is basically a plural form of thu).
Now my question is: based on what those who started to speak in plural for singular assumed that it is respectful to speak that way? In other words, what does make the plural form more respected when talking to him plurally?
Wikipedia has a good summary of the T-V distinction & the various strategies used across different languages. The singular-plural distinction is just one strategy, and not the most common one.
The idea is to emphasize a distinction or distance between speaker & listener.
For example, a person with more social power may be addressed in the plural, since plural is more "powerful" than singular, using the metaphor of multiplicity = power. This leads to forms like French vous, English you and 'royal' We.
Or a more powerful person may be addressed in third person ("your Grace") since that increases distance. This leads to forms like Spanish Usted < vuestra merced 'your grace'*; Portuguese o senhor / a senhora 'sir, madam' < Latin senior 'elder'; Polish pan / pani / etc. 'Lord'.
These strategies change over time to reflect changing social norms. A desire to de-emphasize power differences might lead to diminished use of formal pronouns, or to the reverse: the application of formal pronouns to all listeners - in most varieties of English, "thou" is no longer part of the lexicon.
Concerning the continental european situation:
Addressing an officer in the polite plural can be understood as addressing the office, implying that the actions of the officer are expected to be in accord with their responsibilities, and mostly free of personal value judgement. Consequently, there is--in German or French at least--the same honorific used for actual plural addressees, and the honorific appears impersonal and thus potentially impolite in personally intimate conduct, and offering the 2nd p. sg. between e.g. collegues is honorific in itself.
That describes only a subset of the usage, though.
The primary distinction is made only after kindergarden, in primary school, where teachers are addressed in the plural 3rd p., but the individual pupil is addressed regularly in the singular 2nd p. and the whole of the class is addressed regularly in the 2nd p. pl. It is thinkable that this stems from teacher positions in grammar schools being taken by officers, but this does not matter in a synchronic analysis. Especially children still learning the language may simply take the contrast for granted.
The diachronic analysis might have a long historic tail and vary by language, culture and domain. In many domains the direct address to an authority is taboo-ized, arguably so if imperative forms would start with the nominal. In this sense, the idea of distance as mentioned by @MarkBeadles is often a very real physical distance, that is so ingrained that it prevails after approaching closer. To conclude, note that even the 3rd p. sg. has been used as form of respect (and consequently in sarcastic disrespect).
I like to think that when it was found that in battle an armored horseman was worth 100 soldiers on foot, a nobleman considered himself worth 100 farmers, and hence a farmer had to address a nobleman in the plural, whereas the nobleman would speak to the farmer in the singular. Among their like, people in high ranks would then use the plural, those in low ranks the singular. That is what we have in Shakepare's pieces. Nowadays all English speakers always use the plural, and they are not even aware "you" is not the singular.
@JeffZeitlin regarding German: historically, there were various ways of polite address:
the second person plural, as in English and French: "Seid willkommen, edler Herr, was ist Euer Begehr?"
the third person singular, to keep the distance: "Lass Er sich sagen, Er ist ein Lump!". Since no 3rd person imperative exists, the subjunctive is used instead.
the combination of both, the third person plural.
The third is the polite address in today's German. The verb forms of the 3rd person plural need to be used ("sie sind" = they are, "Sie sind" = you (polite address) are), and the subjunctive replaces the imperative.
