In English polite form of address is "You" which is second person singular and plural. In Russian it is "Вы" which is plural second person.
In Spanish (and probably French and Italian) polite address is the third person.
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Usted comes from Vuestra Merced (later Vuesarced), meaning "Your Grace". Since this was an indirect way of addressing someone, it was inflected in the third person. That is, strictly speaking, you are not addressing the person, but "Their Grace". As time went on, the person inflection was kept, even though its origins became opaque.
In a study entitled El desarrollo de las variantes de vuestra merced a usted, the author shows how this form of treatment developed in the 14th century. Some of its early recorded uses are:
(1) "Señor," dixo el, "yo vos lo dire: "Este preso que se agora partio delante la vuestra merçed, es mi amigo, e fuemos criados en vno [Zifar, 31.9]
(2) E un día el caballero dixo: "Señora, ¡oh qué fermosa sortija tiene vuestra merçed con tan fermoso diamante! Pero, señora, ¿quién uno vos presentase que valiese más que diez, vuestra merçed amar podría a tal ombre? [Corbacho, 146]
(3) E quando vieron que el non los queria oyr, preguntaronle assi: -Sennor, pues vuestra merçed sabe que vuestros enemigos que estan a ocho leguas de aqui, e vos non los queredes aqui atender en esta vuestra muy noble çibdad de Burgos [Pedro I, 148.54b]
"Vostede" (galego) & "vusté" (català) & "você" (português) all come from the same medieval expression "vo(ue)stra/vossa merced(e)/mercê," as it was customary in the Middle ages to speak to those with titles, honors or age in the third person (your honor, your highness, your grace).
"Vos" is original to Latin (vous in French & voi in Italian) and is both singular and plural. The Castilians invented "vosotros" (like the Americans invented "you all") to distinguish between the singular and the plural meanings in the pronoun "vos," which is still commonly used in many countries in Latin America.
The original verb endings for "vos" were "ades"(ar); "edes"(er); ides(ir). In the renaissance era, the "de" was dropped in favor of the tilde (fablades = hablás / habláis) (bebedes = bebés / bebéis) (venides = venís) and the "i" was added (ar/er) to distinguish between the singular "vos" and plural "vosotros" forms. The Castilians in Spain stopped using "vos" in the 18th century while it remains in use in Latin America, where "vosotros" is never used.
"Who is the third who walks always beside you?" (T.S. Eliot, The Waste Land, line 359).
The redoubtable and relentlessly ingenious Luigi Barzini's answer is the most precise, vivid, and memorable:
"The very form of address, the third person singular, is also a Spanish left-over. It is a conventional way of talking not to a man but to his aura, so to speak, to a shadowy person, la sua signoria, his lordship. Signoria being feminine requires all adjectives and pronouns in the feminine gender, even in the case of males, thus engendering suspicions and confusions in foreigners. 'Come sta lei?' for instance, usually rendered as "How are you?', literally means 'How is she?' or "How does this other and invisible person fare?'." (Luigi Barzini, The Italians, New York: Atheneum 1964, 322).