system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: what causes a lisp? What's stopping a person with a lisp simply not putting their tongue between their teeth when they pronounce an "S"?
gpt
There's a thing in linguistics called "articulatory settings", which are the physical behaviours and habits a speaker develops in order to make speech easier and more efficient. Different regions develop different "standard" settings in order to pronounce different sounds from different languages, which is how you get accents when speaking foreign languages -- a Hindi speaker has a distinct accent when speaking English because his articulatory settings don't allow quick and efficient access to the sounds of English, and vice versa. 

Someone with a speech impediment such as a lisp would have just as hard a time consciously altering their settings as this Hindi speaker would. Sometimes they learn over time, sometimes they don't, and sometimes they actively work to overcome their accent. But lots of people, if not most of them, have a hard time doing so, or with mimicking accents reliably, which is why we have speech training. The same applies to those with lisps et al. So for speech "impediments" we have speech therapists who can teach people how to alter their "settings", generally regardless of how those settings became " impeded" in the first place.