The self-protection argument is nothing more than an NRA marketing campaign. It's a scheme to sell guns--has been for, what, 30 years? The chances that anyone would actually need a gun for self-protection are incredibly remote--you're more likely to accidentally shoot yourself or some innocent person. The NRA likes to throw out stats showing huge numbers of cases where "good guys with guns" stopped "bad guys with guns"--but their numbers are totally fraudulent. Bad guys with guns ARE shot and killed--as happened with the Texas mall killer, but it usually happens after they've murdered a half dozen or more people.
The "responsible gun owner" notion is a myth. Once someone buys a guy, he has the potential to become a bad guy with a gun. Why do so many young urban black men carry guns? Answer: self protection! They get guns--most of them--for the same reason you do--
except they have a more legitimate need than some guy in an American suburb. Now, once they get a gun, some of them do stupid things--decide to use their gun to rob somebody of $40 or a bag of weed---and end up dead or in prison, but a lot of them get guns in the first place because they know other people in their neighborhood or peer group have guns. Look at the white gun crazies: They argue even MORE people should get guns to protect themselves against all the other people with guns. It's insane, stupid--uncivilized.
Life circumstances change constantly. "Good guys" get into divorce and custody battles with their wives/ex-wives, get angry and start shooting. It happens a lot. Sacked employees come back to the office with a gun to take their revenge. Multiple cases. Mental problems develop. Look at the Las Vegas shooter---deadliest mass shooting in American history. No criminal record. No documented mental problems--though he'd clearly developed some at some point before killing 67 people (or whatever the number) and wounding some 200 others. No case better illustrates the problem with easy gun ownership and lax regulations.
https://www.cnn.com/2017/10/02/us/las-vegas-attack-stephen-paddock-trnd/index.html