Because I'd be too damned angry to stop myself from using it during moments like this:

Warning, explicit language.

Me:  I'm not angry with you for taking my car.  I am angry with you because you didn't ask. I had no idea you took it and when I tried to leave to run errands, my car was gone. (I  hate driving his car because its a jalopy that is never cleaned out and it has springs poking out in the front seat that always rip my clothes.)

Him:  Well, I took your car for a reason.

Me:  You were gone for two and a half hours! You went to get tickets and you stopped at the mall to play Soul Calibur. I don't mind you taking my car, JUST TELL ME THAT YOU'RE TAKING IT! (Am I talking to a fucking teenager?)

Him: Well, I'm sorry for taking your car and getting you gas.

Me:  What the hell kinda apology is that?  Why don't you try apologizing for real this time, because you know I'm not mad at you for putting gas in my car.

Him: silence......then finally - Okay, I'm sorry for taking your car without your knowledge. I'll just take mine from now on. I won't use your car anymore. (passive aggressive bullshit.)

Me: feeling a vein pop out of my head and steam coming from my ears. Ya know what, nevermind. I'm sorry your ass can't make a decent, muthafuckin' apology!

I was LIVID! Not only did he not give a shit about inconveniencing me, he didn't even care to give me a decent apology when he KNEW he was wrong. I don't mind him using my car at all, I just don't like when he doesn't tell me.  Why is asking someone to let you know in advance when they're going to take your car too much to ask? And before some of you go into in your "when your married you share everything" logic, let me just say that we did NOT purchase that car together, it was mine before we got married and I continued to pay for it until last year when it got paid off.