Why is swearing a sin?

Q.
So yesterday I was having a conversation with a friend in one of my classes about swearing. He's an Atheist with this habit but he was really open to being told why it's wrong to do so, yet I still found I couldn't explain it to him. Obviously just saying "because it's a sin" wasn't going to work, but even looking at that, why is it a sin? The ten commandments say not to take the name of the Lord in vain, but they don't actually say not to swear. In fact I seem to recall that there are numerous times throughout the old testament where it says that the Lord swore an oath to different people. I guess the part that I don't understand is what the difference between saying shoot and saying something else is. What makes one word a swear and something you shouldn't say, while another very similar word is perfectly fine to use in it's place?

A. 
I don't actually think swearing is a sin, in and of itself.  Words come and go in offensiveness!  When French people speak about seals, are they swearing? 

I used to work in a group home for kids with behavior problems, and we had a swear jar.  The trouble was, while I had eliminated swearing from my vocabulary way back in high school, I did not regard things like "damn" or "what the hell" as swears.  So often the kids would catch me, and I would have to put in a quarter!  But who gets to decide these things anyway?

I think that it is obvious why using the Lords name in vain is disrespectful.  And by extension, using any Religious word like a swear word, as the Quebecois do-  the question is why do it?

A friend of mine once said (and probably quoted something) "Common cursing and swearing is an attempt on behalf of the inarticulate person to express himself."  When I was the Dean of Boys at Clear Water Academy, I had a student sent to my office for swearing in class.  I challenged him to compile a list of 10 professions where frequent swearing would not inhibit his advancement.  I expected him to come up with garbage man, trucker, etc-  professions that are respectable in their own right but which I knew he did not aspire to.  But he came up with actor, prostitute, drug dealer....

(Not implying anything wrong with acting!)

I find the words that offend me are different from those that offend others.  I consider it profoundly disrespectful to call someone a 'retard' or to say 'that is so gay', but don't mind as much terms like
sh-t.  It is an interesting education to young people who use the word 'faggot' to learn that the song "Money for nothin'" by the Dire Straits is no longer allowed on Canadian airwaves for use of the word, even while not only songs but radio hosts use a variety of other words traditionally considered swears!

The long and the short of it is this-  swearing is offensive, and therefore is uncharitable.  It is also considered undignified, and so is below the behaviour which should be expected of a Christian who is to be 'above all reproach'.  I think that Christians do ourselves a disservice by overreacting to swears. 

(At a staff meeting in the group home, we were discussing which words to consider swears, and I mentioned the Lords name.  One of my fellow employees said "We can't even say that anymore?  Jesus Christ!"  I looked at him and said "F--k you."  He laughed, and I think that response was probably the most respectable one I could offer!)

So, swearing is wrong because it is not courteous.  But what is courteous is likely to continue to change!