• this world is not my home

I'm not sure of the answer to this. It is a question that has plagued language learners (and alternately amused them) for ages. I think part of the problem is that bad words often have vowel and consonant combinations that are similar to polite, normal, everyday use words.

I remember one instance in particular. I learned the Arabic word for...  maybe I shouldn't tell that story.   

An another thing, why is it that when people learn the words for various parts of the anatomy or bodily functions, they always learn the crude, slang ones first?

For example, I still don't know the non-rude Arabic word for fecal matter, so I can't complain too much. Sigh. It must be an innate problem of human nature. (runs off to look in a dictionary) Okay, I looked it up. I've never heard that word used once in all the years I've been in this country.

I'd say that's probably the reason The proper words just aren't used as often, because they don't have as much emotional content behind them. When we utter profanities (I refuse to call them bad words), it's usually to express a more intense form of an already intense emotion; anger, frustration, sadness (though not usually), happiness and joy. Using these words is more emotionally pleasing because it lets us exude more passion, at least in social contexts. Profanity in literature is quite different.

That's my take on it anyways.

1) They are emotional, like mentioned above.  Emotional things, both good and bad, tend to be eaiser to remember.

2) You can hear them a lot.

I just had another thought about this. When you use an obscene term accidentally in a sentence, everyone around you reacts, often quite strongly with that quick and strong inhale sound, "Hhuuuh! Don't ever say that! That is NOT what you meant!!"

We tend to remember things we learn in these contexts.

Absolutely. That's really the line along which they develop. Think about it; these are supposed to be terms to express the most extreme of emotions. For humans, the most important part of expressing ourselves is that other humans perceive and react to it. If they were words like any other, then they really wouldn't get a jarring reaction save from variables like the tone and volume of your voice.

The words are all usually derived from activities or subjects considered taboo at the time; there are a lot of sex-related profanities that, interestingly enough, are becoming less and less profane with each generation, as the subject grows to be less taboo and more approachable. Same with fecal matter; at various stages in our history, it has been considered a vile thing, a symbol of evil that has been cleansed from the body.

It's interesting to think about what the state of profanities will be in 20 or 30 years.

I'd say that it's down to humor. For me nearly all the words I've been learned are swear words followed by being laughed at. When I'm taught by a girl they a normally actually useful.