I had this one explained by some shrinks once... they weren't my therapists for that matters, just their occupations, my friend just happen to be one. We got into a discussion at dinner, about how people emote. Here's the basics of what they agreed on:
Swearwords are forbidden when you're growing up. You hear them all the time through the media/school/friends but as soon as you said them in front of your parents or other grown ups you got a slap on the wrist or a long boring lecture on how those words are "bad", "evil" or in some way just not right to say...
This makes them very interesting, like cigarettes or alcohol, you're not allowed to use them and you of course wonder why. Then you grow up and all of a sudden you feel like your parents cannot tell you what to do and what not. Swearing makes you feel in control because your "mean mother" who "never loved you" (or whatever) is not able to control you using those "forbidden words".
In a way it's the rebellion against your parents (or authorities), while for some people this is just a phase until they finally grow up themselves, others will continue to use them to feel "bad-ass" and in control.
In the end it all comes down to personal preference how you use (and react to) swearwords. While most shrinks agree that people who curse excessively are very insecure, a little swear once in a while doesn't mean that you are a emotional wreck. While I personally feel that swearing is pretty pathetic, I myself am not able to bring some of my points across without a little bitching!