I am originally from a state in the northeast where it is incredibly difficult to get a gun. I don't know anyone who owns one, except for the odd parent of a friend who had a few guns for hunting purposes (not automatic weapons). I've recently moved south and am around people who are pro-gun for the first time in my life, which has helped me see that the debate is a lot more nuanced than people make it out to be. I have a coworker who is a former police officer and drives around with an "extremely right wing" sticker on his car. He is an avid hunter and supports gun ownership.
However, he in theory supports stricter background checks and was outraged yesterday that the Orlando shooter was allowed to to purchase firearms after being on a terrorist watch list. I've found that a lot of people actually share his views, but there is a fear that if you give in on some aspects of gun control (automatic weapons) it will be a slippery slope and soon the right to bear arms will no longer exist. So while of course there are some gun nuts out there, a lot of it is just simple defiance and bristling at the thought of the federal government slowly trying to eradicate rights that are defined in the constitution. There is also a fear of the federal government having too much control. Because the United States is so large and diverse (the circumstances and needs of a person living in New England are completely different than those of someone living in Alabama) it is very difficult for the federal government to make laws that work for the majority. This is in addition to all of the cultural and historical issues that come into play. The situation is incredibly complicated.
Previous posters have covered the mental health aspect already but I've always thought the rise of social media has played a part in the increase in mass shootings (Gun ownership is actually declining in the U.S. yet we've seen a surge of mass shootings. Peak gun ownership was in the late 70s.). People see all the press these shooters get, and how they are able to draw attention to their "causes". We live in a world now where everyone wants to be insta-famous and the idea of leaving a legacy appeals to some of these disturbed individuals. Access to automatic weapons has made it even easier for them to do more damage. I often wonder what would happen if the US followed the lead of other countries and didn't publicize the names of these murderers.
And as an aside, there are many areas in the US that are incredibly rural, where it could take law enforcement or an ambulance an hour or more to get to you if you needed help. If someone breaks into your home you would not be able to wait for help to arrive. So while I don't think there is ever a need for a civilian to own automatic weapons, there are certain circumstances where it does make sense to have guns for reasons other than hunting. (Gun ownership is "concentrated among older adults, rural residents, and whites, especially white Southerners." Coincidentally, the area where people own the most guns is also the area that feels most strongly about states' rights. The Civil War is still very relevant in the south).