People provide a great photo opportunity. However, you need to be respectful of other cultures as some would not be happy with you taking their photo. It is wise to always ask permission first. Of course it is not necessary if you can take a photo without them knowing, which is easier to do if you have a telephoto lens. Once you ask permission, the opportunity that you saw may have been lost as the subject may now look into the camera smiling or ask their friends to come and join the shot. This photo of three Indian boys taken in Jaisalmer took a long time to get, as once the kids realised I was taking their photo they stopped smiling and waving at me. Luckily they did not mind me snapping away and after many shots I got this one of all three smiling.