Oh, the causes are complex and interrelated. They probably fall into three categories: stressors, feasibility, and culture. You could make a list as long as your arm. Here's some to add:
1. Increased population and its structure, particularly population density combined with transience.
2. Lots of available guns.
3. Violence celebrated as a way of life and approach to life. What are the numbers? Kids see 8,000 murders on TV alone before leaving elementary school. 200,000 acts of violence on TV alone before age 18. (America is bizarre: a woman's bare breast or a man's bare backside gets a movie an automatic R rating. There's virtually no limit, though, to the number of murders you can show and still be PG-13.)
The list could go on and on.
Dulce bellum inexpertis