Once when I heard one man yell at another using words that literally meant "Your mother is a female dog and I want to to mate with you," I couldn't help laughing out loud.
Sometimes perhaps the "bad" words are of Anglo-Saxon origin and the "good" ones are Latin or French based because of the class differences in England during the Middle Ages, but that isn't usually the case. There isn't really much reason for the distinction between good and bad words, it's just a matter of what people agree about.
Come to think of it, that's true of other things, too.
Why should gold be highly valued in so many cultures? Yes, it's rare and pretty, but we can't use it for food, shelter or other things necessary for survival. It made better sense for the Romans to use salt to pay soldiers their salaries.
And money is only valuable because it represents gold but people all over the world now accept it as having worth.
Since humans disagree about so many things I guess its just as well that we do agree about some things, even if only which words are bad.