Unfortunately, these can only be hypothesized. However, by looking at the way humans are influenced in society, I believe there is good evidence to point the blame at several institutions. These include the forces of the media, the government, theatre, drugs and our economic system. On the surface, gangs are caused by peer pressure and greed. Many teens in gangs will pressure peers into becoming part of a gang by making it all sound glamorous.
Money is also an crucial factor.
Although these are important factors they are not strong enough to make kids do things that are strongly against their morals. One of the ways that kids morals are bent so that gang violence becomes more acceptable is the influence of television and movies. Very few hours of television watched by children are educational, so other ideas are being absorbed during this period of time. Many shows on television today are extremely violent and are often shown this from a gang's perspective. A normal adult can see that this is showing how foully that gangs are living.