Mass shootings are mostly, but admittedly not exclusively, an American phenomena. So what factor - or combination of factors is unique to the USA?
It's not caused by violent entertainment - the world is awash with violent media.
It's not caused by neglectful or over-indulgent parents - Europe has no shortage of those.
It's definitely not about 'haves versus have-nots'. Inequality is universal and the perpetrators are frequently among the 'haves'.
Could it be a part of a narcissistic obsession with fame (infamy will do) and celebrity?