Why do people in different places have accents?

Have you ever noticed how people from different places sound different, even when they're speaking the same language? Someone from Texas drawls their vowels long and slow. Someone from Boston drops their Rs like hot potatoes. Someone from London stretches words in directions you didn't know words could go. They're all speaking English — so why don't they all sound the same?
Here's the thing: your accent is basically your mouth's learned habit. When you were a baby, you heard the people around you talking — your parents, your neighbors, the checkout person at the grocery store. Your brain was listening hard, like a spy taking notes. It noticed exactly how they shaped their vowels, where they put their tongue for a T, whether they rolled their Rs or swallowed them whole.
