I think George Orwell describes it well:
The whole business of swearing, especially English swearing, is mysterious. Of its very nature swearing is as irrational as magic indeed, it is a species of magic. But there is also a paradox about it, namely this: Our intention in swearing is to shock and wound, which we do by mentioning something that should be kept secretusually something to do with the sexual functions. But the strange thing is that when a word is well established as a swear word, it seems to lose its original meaning; that is, it loses the thing that made it into a swear word. A word becomes an oath because it means a certain thing, and, because it has become an oath, it ceases to mean that thing. For example, 'fuck'. The Londoners do not now use, or very seldom use, this word in its original meaning; it is on their lips from morning till night, but it is a mere expletive and means nothing. Similarly with 'bugger', which is rapidly losing its original sense.
Words used as insults seem to be governed by the same paradox as swear words. A word becomes an insult, one would suppose, because it means something bad; but m practice its insult-value has little to do with its actual meaning. For example, the most bitter insult one can offer to a Londoner is 'bastard'--which, taken for what it means, is hardly an insult at all. And the worst insult to a woman, either in London or Paris, is 'cow'; a name which might even be a compliment, for cows are among the most likeable of animals. Evidently a word is an insult simply because it is meant as an insult, without reference to its dictionary meaning; words, especially swear words, being what public opinion chooses to make them.