Why is the letter S of the word "sure" - pronounced as "sh"?

Question

Why is the letter S in the word "sure" is pronounced as "sh"?

If we compare most of the words in English (for instance: surface, surname, sum, sir, soup, sun etc.) then we can easily see that the pronunciation of "sure" is an exception.

Answer 1

Very interesting question, the reason is in the etymological evolution of spelling and pronunciation of terms and vowels with a French origin.

Here is an extract from The oddest English spellings, part 18: Why sure and sugar? by linguist and etymologist Anatoly Liberman:

• The vowel occurring in French sure was alien to most Middle English dialects, including the dialect of London, and, as the name of the modern English letter U shows, yu replaced French u in borrowed words.

• We can observe this substitution even in such a recent loanword as menu (and compare nubile and other nu- words). Once sure appeared in English, it turned into syure, and a similar change happened in sugar (syugar).

• Later, syu– developed into sh– (compare bless you, session, and Asia, regardless of whether you have a voiced or a voiceless middle in the last of them, for the voicing is secondary).

Answer 2

'Palatalisation' (or 'yod-coalescence') is what caused sure and sugar to have /ʃ/ instead of /s/.

It's a process whereby the clusters /tj/, /dj/, /sj/ and /zj/ coalesce into /t͡ʃ/, /d͡ʒ/, /ʃ/ and /ʒ/, respectively. It often happens when an alveolar obstruent (/t/, /d/, /s/, /z/) is immediately followed by a /j/.

In Modern English, it mostly affects weak syllables for instance the last syllable in 'mission' /ˈmɪ.ʃ(ə)n/.
The letter 'u' used to be (still in some words like 'sue' in some accents) pronounced /juː/ (the same as the word you), but later on it either got dropped (yod-dropping) or coalesced with the following /j/ (yod-coalescence).

The sequence alveolar obstruent + [ju-] in primary stressed syllables simplified either by coalescence or by preservation of the alveolar obstruent and loss of palatality (loss of /j/).

In the sequence [sju-], the only two items in English with stressed stem-initial [sj-] to [ʃ-] change are sure and sugar and their derivatives (in sure and sugar, the /s/ coalesced with the following /j/ and yielded /ʃ/).
The /j/ in other words like super, suit, suet etc., got dropped (historically) in most accents and only the alveolar obstruent /s-/ remained. So super, suit, suet etc., don't have the cluster /sj/ (or /ʃ/).

Other examples of palatalisation include gradual, soldier, passion, nature, elision etc.

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^{/j/ is the sound as in you.
/t͡ʃ/ as in chip.
/d͡ʒ/ as in jam.
/ʒ/ as in elision
/ʃ/ as in ship.}