Why do dialects develop?

Answer 1

Dialects arise because language changes, for example the changes we see in the comparison of English from 1600 and English today (the words are different in meaning or spelling, different vocabulary sets, etc.). If two groups of people do not regularly (and I mean very regularly) mix, their language will change in an independent way. The changes might be in meanings (or alternative meanings), phrases, sounds (like vowel changes) or any other thing. But because these two groups do not regularly mix and share their language changes, the language changes take their own course and they pile up to give a different feel overall.

But if the two groups can't understand each other so much that their language is almost meaningless to each other, and this might eventually happen, two new languages are formed.

Answer 2

Because the US was once called the "melting pot" because so many immigrants from so many countries flocked to the US with their different dialects and accents. And so it still remains today, different parts of our country have different expressions, accents and ways of talking.

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It's the same in the language's native country even before you add all the contributions from immigrants! A Londoner sounds very different from a Bristolian, a Birmingham resident or a Norfolk native yet these English locations are all well within a couple of hundred miles from London.

I once heard a woman from Middlesborough recognise someone as from Newcastle by his voice. The two cities are only about 30 miles apart and to me (a Southerner living 400 miles from them) they both have much the same, distinctive accent - but locals can tell them apart.

That's even before you start to add Welsh, Scots and Gaelic, although of them only Welsh remains as a living language.