Not only do people like to have a chat, often they also like to take a picture with you. You might feel like a celebrity at first, but chances are you get tired of the pictures after a while. Try politely to indicate that you would rather not have it, but it is probably impossible to avoid it completely.
On the other side: always ask for permission yourself. Usually people like that you want to take a picture. Especially if they are wearing their best clothes, for example at a wedding or ceremony. In some places it is appreciated if you give a tip for the opportunity to take a picture.