Self-Defense is a basic human right. This right naturally includes the use of the tools to assist in that right. The tools in question that allow the best use of the right are firearms. Guns eliminate power differences between different people, and firearms training and weapon possession cannot be evaluated by casual observation, making an attack on a random target a risky and dangerous proposition.
"To train an archer, begin with his grandfather." - Archery Proverb.
At the time of the American Revolution, the English Longbow was a more dangerous weapon than the muskets and rifles available at the time. But while the guns could be used effectively with only moderate training, allowing their use by farmers and frontiersmen, but a longbow requires a lifetime of dedicated training to be used effectively, precluding archers from having another position or just being an archer "on the side". On simple logistical grounds, it was easier to train, field and replace large numbers of riflemen than it was to do the same for archers.
A similar concept holds true today - someone who commits themselves to physical training is going to be stronger than someone who doesn't. For people who intend harm to others, being strong and powerful matters. For people who have no such intentions, it generally doesn't matter. But effective firearms training doesn't require the same level of dedication, but gives serious power in spite of the lesser training.
In fact, guns make strength differentials irrelevant. A five-foot two, hundred-pound woman is just as dangerous as a six-foot seven bodybuilder when both have a gun. And with a bit of training, which is easier to get when you aren't a crook, the woman would be more dangerous than the thug.
And that's a major reason women are leading the concealed-carry movement. Women who've been the victims of stalking or violence understand how effective a gun is, even just as a deterrent. A stalking victim who carries a gun goes from target to threat in the eyes of her stalker. Or her would-be rapist, or kidnapper. Nothing says "Run" like looking down the barrel of a pistol.
And that shifts the incentives. Criminals in areas with heavy gun control operate more aggressively. They actively target occupied homes to rob, because the owners can be compelled to assist the robbers in finding valuables. Street thugs can rob people in broad daylight, as the UK government once admitted when they warned people not to use electronics while walking down the street, in the middle of the day.
But in areas that are gun-friendly, criminals actively avoid encounters with homeowners or in crowded areas, as there was a possibility that someone else may have a gun. A thug in, for example, Houston, does not know if the person they are targeting is armed, or if someone nearby is armed, enhancing the costs and risks involved to the thug, often beyond acceptable limits.
Likewise, even mass shooter actively seek out gun-free or gun-restricted zones to commit their shootings. But as soon as these shooters are confronted by another armed individual, they either surrender or commit suicide. There is no instance where a mass shooter ever engaged in a shoot-out with an armed bystander, or even attempted to continue shooting unarmed victims.
There are also no upticks in crime or violence after the passage of shall-issue. CCW holders have rates of criminal activity below that of police. Importantly, there are small-but-significant drops in crime, despite the substantial social pressure to not carry.
In short, there is no reason to disallow a general concealed-carry regime for self-defense, and many reasons to allow it, even disregarding the legal and Constitutional issues involved.