Do You Speak American . Words That Shouldn’t Be? . Sez Who? . Change | PBS
Language change is a very interesting thing to look into. Languages change due to political and social forces. When people start speaking differently, they still know what is the “correct” (correct is defined only by usage, not by some textbook or dictionary) form that they grew up with but will still choose to change how they pronounce some words or phrases.
The next generation will have grown up with those changes so to them they are normal. The cycle will then repeat in that they make changes to what they feel is normal language. Thus over the generations the language will change more and more. Languages tend to change more slowly in literate societies since people are connected to what the language was like in preceding centuries.