Let's bury a common language myth today: the true origin of the Spanish "lisp." Where Latin America and parts of Spain pronounce [s], most of Spain pronounces as [θ]. The legend goes that there was once a beloved king that had a lisp, and so beloved was this king that his adoring subjects imitated his lisp, and that lisp caught on and the Spanish language changed forever.
Before we talk about why this isn't true, I want to define some of the terms we're going to be using. Spoken sounds are represented between [ ] or / /, while written letters are represented in greater-than and less-than signs, < >. Sometimes a written letter matches the sound, such as the letter <s> in English matching the spoken sound [s]. Other times it doesn't, such as <ough>: in thorough it represents the sound [o] while in rough it represents [əf] (that UFF sound). Here are some of the symbols we'll be using for spoken sounds:
[s] = less ; [θ] = think ; /t͡s̪/ = tsar (but not czar) ; /d͡z̪/ = closest to adz
Returning to the legend of the Spanish lisp, not only was there no such king on record with a lisp, but the tale doesn't really make any sense. First, a lisp makes every ocurrence of [s] into [θ], but the Spanish accents don't lisp every [s] sound. They only lisp instances are where the letter is <c> or <z>. Letter <s> stays an [s]. If they were lisping the [s], then why did only <c> and <z> change? Lisps don't operate selectively.
The truth of the matter is that the Spanish letters <c> and <z> were neither [θ] nor [s] originally. Both are independent products of old sound changes. The letter <c> stood for /t͡s̪/ and the letter <z> for /d͡z̪/. (The letter <s> has always stood for /s/ and hasn't changed).
As Spanish evolved over time, in most of Spain, /d͡z̪/ and /t͡s̪/ merged toward /θ/, so the letters <c> and <z> stood for [θ] while <s> stood for [s]. In southern Spain, /d͡z̪/ and /t͡s̪/ merged with /s/. (There are actually tiny pockets in Spain and the Canaries that took the sounds in different directions but that's outside the scope of this post). So in southern Spain, <c> and <z> and <s> all stand for the same [s] sound!
When Spain began to colonize the New World, Latin America was mainly populated by immigrants from Andalucia - the southernmost region of mainland Spain where <c> and <z> and <s> all stand for [s]. The Andalucian accent took root in Latin America and is responsible for the [s] that Americans think is "normal." In fact, all of them are normal products of older sound changes.