Why are people becoming desensitized to human suffering?

Sometimes it seems like the majority of people can't really empathize with others. It saddens me to see how people (myself included) can look at a picture of a starving kid one minute, and then go lavish themselves with expensive stuff the next. I still remember a few years back when we were learning about the holocaust, all the kids in my cl (9 year olds) were totally OUTRAGED. Some even started to cry. But now, we just don't care. I myself have experienced this shift of values too. When I was like 10, I would dream of rescuing some poor kid and take her home and feed her and all that. I used to feel so sorry for beggars. But now I'm just a b*tch. I hate my parents and myself and am obsessed with looking beautiful. Seems like most people I know are like that too. I know it's happening, but the most important thing is, WHY????