Because I can.
Because I should.
To protect my wife, myself, or other innocents.
Here's the reason (I posted this on another thread):
Approximately three-and-a-half years ago, I was mugged outside of my apartment complex late one Saturday night. I was carrying a bundle of suit jackets and ties (I had been shopping that day) among other things when I noticed a car pull up a short distance behind me: I initially assumed that someone was dropping off a friend. I began to walk on, but something inside told me to look over my shoulder: I saw a man running toward me with a gun pointed at me. He was a car length away, but in such a position that I made a split second decision to drop everything and run for my life. I ran, instead of obeying him and giving him what he wanted, because I had heard of a number of incidents where muggers simply shot their victims and took what they wanted. I also heard a police officer on a radio show state that if a mugger tries to force you into a car (to take you to ATM machines, for example), you should either fight or run: if you get into the car, you will be killed.
As I ran, I cringed in fear as I listened for shots being fired, expecting to be hit in the back. The mugger did not shoot; however, I had no idea whether or not he was pursuing me. Since I was running at top speed, I soon ran out of breath. I cannot adequately express to you the fear and utter helplessness that I felt as I realized that I could absolutely run no farther and turned to see if my attacker was behind me. If he was behind me, I had no strength to fight or run further: I would be totally at his mercy. I thought briefly that my loved ones have no idea what’s going on right now. Fortunately, he did not pursue, and I escaped unhurt while only losing some new neckties. I read subsequently in the local newspaper that approximately 20 similar incidents had been perpetrated by what police suspected to be the same pair of suspects over the same weekend: victims who had not been as fortunate as I had had guns pointed at their heads and were told repeatedly that they would be killed. Thankfully, none of the victims were killed.
I and my wife now each own a handgun and have our CHL. We practice regularly and are prepared to use our guns in self defense.