Quite a few of my compatriots have written on this topic lately, and I'll link their eloquent responses at the end of the post. They got me thinking about it, too.
Many who know me think of me as a "gun nut." If they have a question about a gun, they come to me because I either have an answer or know where to get one. They don't, however, necessarily agree with my views on guns. On more than one occasion, people have said I'm paranoid.
I don't wonder whether that bush is the hiding place for some mugger. I don't think every creak in my house is a burglar. I'm prepared, though, for the possibility of something bad happening. You don't lock the door out of paranoia. You don't put on a seatbelt because you plan to crash. You don't carry a firearm because you expect to need it.
Others joke that I'm eager to kill. They know (I hope) that I would prefer not to kill anyone. I would rather be the one still alive should it come down to kill or be killed, and I also know that the presence of gun can be enough to diffuse some situations.
There are those who carry and assume they'll be ready. I know someone who has a pistol ready in case things go bad, but he admitted to me that he has never fired it (this same gentleman has a high-end 1911 he regularly fires, but is afraid to use as a carry gun, in case it should be taken away). I take my XD to the range every time I go (no matter which guns I plan to fire, I fire it, too), and I stay in practice. I don't do this because I expect to need the training, but because it would be best to be prepared. Sun Tzu wrote that "The art of war teaches us to rely not on the likelihood of the enemy's not coming, but on our own readiness to receive him; not on the chance of his not attacking, but rather on the fact that we have made our position unassailable." Or, as the Boy Scouts put it "Be Prepared." Remember that the responsibility falls to Just. One. Person. Yourself.
Finally, I carry for those who won't or can't. If there is ever a situation in which I can save a life or two, it's worth it. I don't expect to, but I am prepared for the situation. Sometimes, a part of me wishes I could've been around for the latest shooting. It's not what a reasonable person should think. After all, I'd be likely to be shot. But I'd also be likely to stop the shooter, saving people's lives. Yes, my instinct for self-preservation is intact (I think), but I really hate to see lives cut short because no one was willing or able to defend them. I don't even want to do it because of the possibility of "hero" status. I would expect heavy criticism, in fact. People would say the shooter could've been talked down, others would ask why I was carrying in the first place, and some would undoubtedly accuse me of wanting to kill. But a few lives would continue as a result, and I would know that I had done the right thing. No matter how anyone feels about it, I would know that I had done something right. If I have no other justification for my existence, that alone would be enough.
I don't carry because I want to. I carry because it's necessary.
More eloquent reasonings:
Why Do I Carry? at Blogonomicon
Confessions of a Deathbeast at Hell in a Handbasket
Why I Carry at What Would John Wayne Do?
Last Words on Why I Carry at When Your Only Tool is a Hammer
Just. One. Person. at Joe's Crabby Shack