There are really two mass shooting crisis in the US. One is where the shooter tries to kill as many people as possible before either killing themselves or being caught or killed by law enforcement. The targets are often random. Sometimes the shootings have distinct political motivations like the Walmart shooting in El Paso and the church shooting in S. Carolina. Other times, the shootings are just the sad end of a spiraling mental health crisis.
The other mass shootings are largely gang warfare. These have gone beyond turf battles and have become nothing more than a boiling over of perceived disrespect on social media. What would have been a fist fight resulting in a black eye or bloody nose 30-40 years ago are now gun fights where multiple high capacity clips are emptied.
The reason these mass shootings are rampant in the US is two fold. First, guns of every shape and size are readily available to anyone at anytime. Second, our gun culture views gun ownership as empowerment derived from the warped view that the 2nd amendment was intended to grant every citizen the right to take up arms against what they perceive to be injustice.
Regulation of the sale of firearms in the US is a complete joke. The few states that have strong firearm sales laws are overrun with guns coming from neighboring states that have lax laws. And for criminals who do not even attempt to buy guns legally, the black market is thriving with stolen guns and guns bought by straw purchasers (i.e. someone with a clean record who buys in order to resell to the black market). The result is that anyone who wants a gun in the US can get whatever they want. The only real restriction is on fully automatic weapons, but if you can still get a bump stock and effectively convert a AR15 to a fully automatic as the guy did in the Las Vegas mass shooting.
In just about every other country, there is no right to own a firearm. It is a well regulated privilege with extensive permitting requirements and strict regulations on the kind and quantity of firearms you can own. In most countries, people just do not own firearms for personal protection or as a collection. Guns are for hunting and for a very small number of people who keep them at home for personal protection because they have jobs that make them vulnerable (judge, criminal prosecutor, etc.).
In the US, just about everyone has a firearm and many collect firearms. Gun ownership is not just about personal protection, but is about empowerment. If you have a gun, you get to use it to have the upper hand in any situation you are in. I was out playing golf and when I made the turn, there were a dozen cop cars at the club house. Two groups of golfers got into an argument about one group hitting into the other. One guy went back to his bag and pulled a gun on the other group. Then, a guy in the other group went back to his bag and pulled a gun on the other group. The stood around pointing guns at each other until the police arrived and disarmed everyone. This gun culture comes out of warped Second Amendment interpretation of the mention of arming the states against the potential tyranny of the federal government. Thus, your right to firearms is not just to protect your self from criminals, but to empower you to take on what you believe is wrong in the world. This is why most mass shooters, especially school shooters, are people who are marginalized, bullied and often suffer from mental health issues. They turn to guns as a way to empower themselves against a world that has mistreated them.