Native Americans, including some from here, Brazil, are afraid of photos. They don't like it when you take pictures of them because they think that you are stealing their souls.
Why do they think that?
Well, not many do anymore. I found this on a question on the skeptics stack exchange
[Carolyn J. Marr] illustrates a change in Native Americans' attitudes towards photography from the late 19th to the early 20th century.
At first, many Native Americans were wary of having their photographs taken and often refused. They believed that the process could steal a person's soul and disrespected the spiritual world.
Over time, however, some Native Americans came to cherish photographs as links to ancestors and even integrated them into important ceremonies.
Source: Historical Analysis and Interpretation: Photographs and Symbols
As to why, the indigenous people didn't understand this technology. In their minds, they saw this as magic that could trap a person or an image inside something. Hence the idea that it would steal your soul. This is similar to the ideas of medieval peoples that found alien things magical and often evil (hence the witch hunts, even though they weren't as huge as they seem), I'm sure the reaction of a medieval person to photographs would be very similar.