There has been some interesting comments so far, and some, no doubt, that actually apply to the state of our society today, but the answers you are looking for are not simple, nor of a single entity.
I know you said you didn't want this to turn into a "then vs now" thread, but you cannot entirely escape it, because we went from where we were to where we are now, and that, mostly is the why you are looking for. It has to do with the "how" we got here being the answer to the "why" you are looking for.
The single most significant event of the last century was the Second World War. The ideas of the combatant nations, the technology developed, and the social changes that occurred as a result of the war set us on a path the likes of which we had never seen before. Many ideas mentioned by other posters can be traced directly or indirectly to the war, and what we did to win it. Some of these results (in no particular order)
The emasculation of the male - A combination of factors, acting over time, and building on each other. The horror of the war, the first in history to actually achieve "total war" (even though the US was spared from combat), cities bombed into rubble, millions of "innocent" civilians killed. We did this to our enemies, who returned the favor when they could. It put a huge idea into the collective subconscious, that war was bad, war was evil. prior to this (actually starting with the horror of the First world war), war was not considered evil. Bad, for those who fought, and those caught in the fighting, but not evil. Militarists were respected, and were powerful men, being high in the governments of most nations. There were no superpowers, no world cops to force nations to abide by the rule of law, only military might was respected. Only military might kept your nation free from invasion by neighbors who would gladly take what you had, for no other reason than the wanted it, and the could. Military traditions were respected, and young men going into the military was an honored duty. We still keep some of those feelings, but others have changed.
Our enemies in WWII were blatant militarists, and the horror of the war was seen as a result of the militaristic mindset. Add to this, the view of some that men caused this. Militarism became seen to be bad, if not outright evil, and other traditional male values and behaviors became painted with the same brush. The tremendous social changes brought about by our having to place women in the industrial workforce in large numbers to produce the arms for our men to use to win the war empowered women in a way they never had before. The idea that women might actually be equal to men became not just a philosophical concept, but a practical ideal to be sought. And over time, women have gained full legal equality, and made huge strides toward full social equality as well. But along the way, traditional male values and behaviors have suffered. This is not altogether a bad thing, just one of the things that have changed our society.
All our traditional values have come under fire in the past 60+ years, and some have stood the test better than others. Many ideals have been changed, and many for the better, but sadly, some for the worse. America is closer to being the nation of equality our laws have promised us than ever before, in many areas, but in others, we have lost things that were once taken for granted.
Another one of the things that got us where we are today is violence, and our dealing with it. Human society involves a certain amount of violence. Until human nature changes, this will be so. How we deal with the violence goes a long way in determining how stable and orderly our society will be.
Well intentioned people have been conducting a great social experiment with us as the test animals for some time now. In the name of reducing the violence in our lives, we have made virtually any show of agression or violence (including necessary and justified ones) into crimes. People got into fights, people argued with their wives, people punched out guys who insulted their dates, men even got slapped by women for improper advances. All without the police being involved, or anyone being arrested and going to jail. Not so today. Every small thing has been made into some level of crime, here in what we still call "the land of the free". Ironic. Punks robbing a convienence store shoot the clerk (no witnesses), because they get the same punishment for the killing as they do for the robbery, IF they get caught.
DESENSITIZATION - look at our entertainment, full of violence, each year more and more. We become jaded, bored, and in order to sell us their product, the entertainement industry constantly goes further and further. There is no check on this (First Amendment, don't ya know), other than the free market, and the free market always goes to the lowerst human behaviors, as long as we are willing, and able to pay. Movies and TV, constantly show little that isn't violence of some sort, and ususally gunplay. And horribly unrealistic at that. Our popular video entertainment has been teaching us for decades that the way to solve problems is to use a gun. And because the actors don't really get shot and die, the same faces show up again in other shows, teaching us subliminally that using a gun on someone isn't really a permant thing. Think about how many shootings our children see, before they have a firm grip on the reality of the world (and some never do get that firm grip). Thousands of shootings, right in our homes, day in, day out, 24/7. Most of us can deal with it, it is just make believe, right? But there are those who can't.
Another thing that has changed our society, and one that has so far not been mentioned is CREDIT CARDS! Not just the cards, but the whole consumer credit industry. We have gone from a nation where hard work and saving was necessary to get the nice things in life to a nation where we are encouraged constantly to get it now! You can pay later. Instant gratification, and as one result of this, it has become not only expected, but demanded. People can get everything without working for it beforehand, many have come to believe it is their right to have it all, and have it without working for it.
Religion had become one of the whipping boys of the intellectual elite. religious people are ridiculed, told their beliefs are old fashioned and irrelevant. There is no Heaven, no Hell, only today, and all that counts is how you make it today. Or so we get told. Over and over.
I do not wonder that cultures other than our own, who get their information about us solely from TV, movies, and American corporations out to make a buck, find us so easy to hate. Greed, materialism, and the complete disregard for others is celebrated endlessly in our popular culture, and promoted shamelessy wherever our influence reaches. Sad.
Not all of us are like that, but too many are.
I have named a few of the things that I think have contributed to us getting where we are today (and only in really general terms), but not all. Hope this sparks further discussion. remember, bad as it is, it can still get worse. But don't give up hope. Giving up hope is what ultimately leads those troubled individuals into committing the shootings.
