My personal theory is because we - as a society - no longer teach respect and personal responsibility to our young. Schools, government, TV, and other media teach the young that 1: violence is prohibited; there is no "good violence" (e.g. self-defense); bullies are misunderstood and need to be reasoned with instead of having their butts kicked until they behave; no need to respect others - if you want a vacation from school, beat up your teacher - - that's what the schools teach.
2. Government teaches them that no one is responsible for his lot in life; that no one should ever have to suffer the consequences of making bad decisions; that it's somebody else's responsibility to keep YOU safe; and that government - and no one else - is competent to look after you.
3. TV and other media sell the concept that violence is cool and edgy, play up the notoriety of bad guys and downplay the consequences of committing violence on others, and point out all the little inequalities of life which must be blamed on someone.
Finally, our lives are becoming so regimented, so constricted, and so conflicted that some people just feel the need to strike out at SOMEONE. Since firearms are plentiful in this country - as opposed to other countries where civil war and revolutionary violence isn't endemic - it is simply more convenient to kill folks with a firearm than by other means.
Don't forget, we still kill more people in this country by the use of automotive transport than by shooting them. Also, don't forget that folks who really want to kill people still manage to accomplish it by other means.
