Why do languages develop different accents?

Answer provided by Anthea Fraser Gupta (With input from other panelists), School of English, University of Leeds
Human nature. In all sorts of ways, we behave like those we mix with. We are members of social groups, and within our social group we like to behave in similar ways and show that we belong. We do this in language as well as in other ways (e.g. what we wear, what we eat).

When groups become distinct, the way they speak becomes distinct too. This happens socially and geographically, but is easiest to illustrate by geographical differences. If a single group splits into two (imagine that one half goes to Island A and one half to Island B), then once they have separated, their accents will change over time, but not in the same way, so that after just one generation the accent of Island A will be different from the accent of Island B. If they stay completely separated for centuries, their dialects may become so different that we will start wanting to say they are speaking two different languages.

Humans like to travel. Since humans left their place of origin in East Africa, more than 100,000 years ago, they have spread all over the world. And they have moved in waves in some places, mixing with, or conquering, people who were there earlier. One of the last places humans reached was New Zealand, which Polynesian people (now known as ‘Maoris’) settled in in the fourteenth century (CE), joined by Europeans four hundred years later. English developed in England as a result of people moving to England from across the North Sea in the fifth century (CE) — they were at least the fourth major wave of humans to reach the island of Britain, and the descendants of the previous waves were still there when they arrived to mix with them. In modern times (the last 400 years) the activities of aggressive and acquisitive Europeans has resulted in them moving all over the world and taking their languages (especially English, Spanish, Portuguese, Dutch and French) with them.

Source: http://linguistlist.org/ask-ling/accent.html