Foul language is treated differently by the brain than other "acceptable" words. From (usually) birth, a child is taught that "this" word and "that" word are forbidden. Although these are words are included in every language, they are still socially unacceptable. Why create a word if it is to never be used? Now, I'm not advocating the excessive use of profanity here, I'm just asking why create the word if you have no intention on allowing it to be used? Young children are told over and over not to talk like mom or dad, so the brain associates "Never...EVER...say this!" with many "foul" words (that I feel no need to list). Swear words are treated differently by the brain because they most often represent strong feelings. So, instead of describing a feeling with a word, the feeling is represented by the word and the tone in which the word is said (i. e. "damn" could represent disappointment or even emphasize approval). For many people, saying these words relieve stress. No word is better to release/describe your anger, sadness, joy, etc. than a word that is taboo.

Since English is a living language, (as opposed to a dead one. i. e. Latin) the definitions of words are always changing. Many previously acceptable words are now associated with derogatory, racial, and generally negative "no-nos." Remember when being "gay"1 meant you were happy or that stupid "ass"1 was just a not-so-intelligent donkey or even when a "cracker"1 was something you ate?

1: These words may not exactly be "forbidden," but they are over-used (or even actually used i. e. "gay") as derogatory terms.