POLL: Why do mass shootings happen more in the US than the rest of the World?

The United States has the highest number of mass shootings per capita in the Western World, even among countries with high rates of gun ownership. Gun control advocates claim that this is due to the number of guns and the lack of laws preventing access to high-capacity weapons. 2nd Amendment supporters, however, say that it is due to cultural factors and mental illness.

Why do you think mass shootings occur more regularly in the US than the rest of the Western World?