The answer to "why" is often elusive and dependent upon one's moral, political, religious, social and racial background. Some blame mass shootings on immigrants and terrorists and others focus upon mental health as the leading cause. A recent study claims that 54% or mass shootings in the US are related to domestic violence. Other causes often cited include, being fired from a job, being on the receiving end of a restraining order, divorce, being cheated on, sexual frustration, or some other reason stemming from being aggrieved. Further, the ease of access to firearms is also cited as a factor especially if one is available close to the time that the person was aggrieved. You know, you cheated on me and I have a gun in the dresser type thing. And, of course, there are those who love to blame those who are different for any and every ill in their life. "I lost my job, those damn Muslims are to blame, think I'll grab my rifle and get rid of some of them." Many also effectively argue that the mentally ill cause very few mass shootings and others will point to racism as a major cause of this social aberation. Clearly, killing people while in church, at a concert, or killing any place is not a sane act in the moment.
A question that needs to be addressed is, Why is mass shooting becoming so prevalent in the US? I think we have to go beyond the why's in any given case and look to see if there is an underlying cause. I think we have to look at the American Culture of Violence. Our history is quite violent and we have often resorted to the gun to solve the problem. We are a gunslinger culture. Kill what gets in your way. Native Americans, kill'em. Wildlife, kill'em. Competitors, kill'em. From the lore of the wild west to the allure of the mobster, Americans love to see the enemy get their due at the end of a barrel. Okay, we have become more "civilized," and we generally don't shoot someone every time things don't go right, but that doesn't mean that the desire is not harbored deep within. Afterall, the most popular movies and games are those that have high degrees of violence. So much so that many have become anesthetized to the killing.
Any plan to take guns away is doomed to failure. Certainly, under a more liberalized political system, changes can be made to rules and regulations that will reduce the likelihood of a mass shooting. Certainly, better screening, vetting, can be done on those who seek firearms. Certainly, there can be better coordination among agencies that keep track of potential shooters or disorders that may lead to shootings and to prevent access to firearms. But here's the thing, when the leader of a nation or any idealogue awakens and supports the gunsliger mentality, bad things, very bad things, are more likely to happen. When a leader mimmicks the shooting of a rifle to kill someone, that action enables the gunslinger mentality. When movies, games and TV shows resolve most situations by killing, the culture of violence is nourished. When religious leaders or rather religious radicals promote attacks on "them," contribute to hate speech, and condone bigotry, then the culture of violence is enabled to rear it's ugly head.
Until and unless we end the fixation on violence to solve problems, all the laws, rules, regulations, and prayers will be for naught.