The thing is, quite simply, that Americans love guns too much. It's just your gun culture. If a high enough number of people likes guns, wants guns and owns guns, there will always be some people who will get and use guns inappropriately. Those people may be unbalanced individuals that go on shooting sprees, people who don't store guns away from children, people who lose their guns, people who don't maintain their guns regularly, people who sell their guns on the black market, or whatever else you can think of. These people will be a very small percentage of the overall group, but as that group is huge, they will still be significant enough to cause stuff like that happens almost everyday in the States.
And forget about changing this. Outside of some sort of catastrophe, a society's basic cultural trait usually changes only over decades, at the very least. If something like Sandy Hook didn't change perspectives towards guns, then nothing will.