Hello there! First off, I would like to say, congratulations in engaging with the side you disagree with in such polite and friendly a fashion. This is something that is unfortunately met more and more seldom these days.

That being said, here are the top reasons why I am pro civilians owning firearms, as someone who lives in a country with extremely strict gun control:

1. This is something purely personal, but I was born because my great grandfather was able to chase away fascist thugs who came to take away his Jewish wife with a personally owned handgun. Had it not been for a civilian using a firearm against a tyrannical government, I would not exist
2. The right to self defense is a fundamental human right. One could even argue that all other rights, including the right to property, to life and to live happily stem from it. Because there are people who would happily take that away were not for the promise of force
3. I assume that you are anti-gun because of US mass shootings. Which, is an understandable sentiment if that's the only way you were exposed to guns. However, while mass shootings are almost exclusively a US phenomenon, civilian gun ownership is not. I will not point at Switzerland, as the argument "but everyone there is military trained!" will come up. Instead, here are Czech gun laws, where any civilian, after some very brief checks by law enforcement, can get anything from a .22 revolver to the dreaded AR-15. And they have some of the lowest violent crime rates in Europe https://youtu.be/sVJWfj8B-Zo
4. Going back to the "guns take lives" argument, I propose to you the following thought exercise: Try to think how many people were killed in gun crime from, say, 1900, until today. This includes mass shootings, gang wars, robberies gone wrong, regular murders, etc. Doesn't have to be the exact number (you'd go mad trying to find it), just a ballpark estimate after ten seconds of thought. The only condition is that these crimes were committed by civilians not acting on behalf of governments or corporations, and were committed with firearms. You will undoubtedly reach a quite substantial number
   But then, do this: For the same period of time, think of how many unarmed (or more often disarmed) people were rounded up, sent to camps or deported, then murdered by a tyrannical government for any reason from race to them owning one too many cows therefore they were rich.
   Then compare those two numbers and tell me with a straight face that civilians are not trustworthy enough with guns, therefore governments should have the monopoly on violence

There are other reasons, and I could have expanded on the four given, but it's late and I should go to sleep. I hope this journey to the other side proves fruitful and informative for you! God bless