When international friends hear me talking German, they always think I must be really angry and having an argument with somebody.
What are the phonetical explanations for making the German language sound so harsh or rude?
This is a factor because consonants are often perceived more harsh than vowels.
German is a very vowel-rich language. There are reasonable vowel definitions containing 23 of them (see, e.g., this list) plus three diphthongs¹. Consequently, vowels have a high information density,² we do not have to use that many of them, which in turn leads to a high consonant density. To somewhat exemplify this, consider words like zwingst, blinkt, strickt, hältst.
Relatedly, we rarely have vowels in directly following each other. This almost only occurs in diphthongs, of which we have only three¹, and when a word stem ending on a vowel is followed by an inflectional morphem beginning with a vowel such as in säen or schreien. (See below, if you think that two vowels follow each other in words like beerdigen.) This is intertwined with the high vowel density, as direct vowel collisions would inevitably lead to some diphthongisation, which would make the distinction of words very difficult (23 vowels are already difficult to distinguish). Again, this leads to a high consonant density.
¹ not counting vowel + r and ui, which only appears in a very few words.
² See this question for a demonstration.
This is a factor because devoiced consonants are usally perceived more harsh than voiced consonants.
A main factor for this is certainly the already mentioned phenomenon of terminal devoicing (Auslautverhärtung), but there is more.
Voiced consonants are very unlikely to follow a short vowel. Just think of how few words there are which are spelt with bb, dd, gg, vv or ww. Moreover, ss does not indicate a short vowel preciding a voiced s, but an unvoiced s.
Finally, the voiced counterpart of the sch sound (ʒ) occurs only in a few loanwords such as Garage. So, we are short of a voiced consonant in comparison to some other European languages. Moreover, many Southern dialects and varieties of German lack the voiced s altogether.
The glottal stop is the sound separating the two e in beerdigen. Every German word whose first letter is a vowel starts with this sound. Many native speakers are not even aware of this relatively frequent phoneme and it’s one of the hallmarks of a German accent in most other languages.
This is a factor because it is yet another consonantic sound (see above) and a particular harsh one: Many ways of grumbling, grunting and screaming employ a variant of this sound.
As @Jan said, the so called Auslautverhärtung definitely plays into that, but if you think in terms of pronunciation and intonation you'll also notice that many languages link their words together in ways that the German language simply doesn't. Take English for example; among other things, words that begin with a vowel are usually linked to the word that comes before them.
"[…]for example[…]" — /fə_r_ɪɡˈzɑːmpl/
In written English you see that those are two words, but if you didn't know any English and you heard those two words, you wouldn't know where the first ended and the second began. That's the kind of linking between words that you don't really find in German and I take it speakers of foreign languages find that rough and perhaps even "unnatural". My English pronunciation professor likes to say, that "Germans don't like their words to touch each other." Think of any German sentence and try it out.
Ich war gestern Abend im Kino.
You can clearly distinguish each word as a single entity. No word ending slides into the subsequent word. Compare that to English and you'll either sound like a robot or like you're angrily forcing each word out at a time. (You're hopefully not a robot.)
Another thing is the pitch of your voice. Obviously voices vary in pitch, but in general you notice that other languages use a much wider range than German does. It makes it sound a little monotonous compared to other languages.
Lastly the probably most obvious thing is the sound /x/ that we use for words like "Kuchen" ([ˈkuːxn̩]). That one sounds awfully harsh, doesn't it?
Wrzlprmft’s answer is great and definitely deserves the acceptance. However, there is another aspect he failed to address: aspiration.
In German, (almost) all unvoiced stops are aspirated, whether they occur word-initial (Tor) word-final (rot) or word-internal (hatte). By comparison, in French or Finnish stops are always unaspirated and in English they are usually only aspirated in word-initial positions. The aspiration often causes speakers of those languages to perceive German as rather harsh and often even perceive a German accent in their own language as harsh.
A milder, second aspekt is the presence of up to all four different affricates: tz, tsch, pf and in dialects kch. While ts and tsch are somewhat known across many languages (and, ironically, tsch is rather rare in German), pf and kch are very rare across the world and only occur in German across the developped countries. Especially the non-dialectal pf may also contribute significantly to perceived harshness.
Alright, I heard that one before :) I am a native German AND Englishspeaker. I have lived for 3 years+ in USA(Elementary School), Germany-Bavaria/Munich(HighSchool, some College),England-London(University), and now Austria-Vienna(Grad-study,Work).
In my opinion, and also from experience, I think the "harsh sounding german" is mostly (not 100% tho) a stereotype.
I think it mostly has to do with the terrible history of Germany in the 20th century:mostly WW-II, Hitler and the Holocaust. This has left the Country as well as the language with a really bad reputation over the years. In Countries like the US, and even England, most people still draw their knowledge of Germany from the many Hollywood Nazi Movies - with tall blonde Soldiers shouting around orders, running into combat like robots from Star Wars (guess who was the Prototype for the Empire), and laughing like evil devils.
I would compare it to the way the arabic and maybe russian is portrayed in Hollywood movies atm. Most Englishspeakers think those languages sound harsh as well.
Now - that being said, many German dialects could sound a bit harsh to the average Englishspeaker even if History had gone down a different path. I noticed that (British, and even US) English is generally more polite than the major northern German dialects. Also, while seeming a little bit rude at times, German is still very formal: The kind of Wordgames, and new Word creations that is common in English, and especially the humor has only started to enter spoken common German the last 20years or so. I have to admit that I sometimes miss the humor and lightness so common in general English conversation when I am in Germany. But it's not the only Country to have that problem, and that is not a general characteristic of the language itself. For example, if you go from Bavaria to Austria,Vienna, people have a more laisser-faire way, and they speak, even though akin to Bavarian dialect, in a more melodic, sing-sang and humorous tone. And even though Viennese does have a lot of peculiarities, it still IS the same (German) language...
Wir müssen unterscheiden zwischen den deutschen Dialekten einerseits und der künstlichen sogenannten hochdeutschen Sprache andererseits.
Die Dialekte klingen nicht unbedingt hart. Sie sind nämlich das gesprochene Wort. Sie sind die echte deutsche Sprache - pluralistisch, natürlich, organisch gewachsen.
Die sogenannte Hochsprache dagegen klingt hart. Sie ist künstlich. Sie wurde für die Schrift entwickelt und auf die Schrift ausgerichtet. Niemand hat früher so gesprochen. Hochdeutsch wurde über die Schule und das Militär ausgerollt. In Norddeutschland mußte Hochdeutsch sogar als Fremdsprache gelernt werden, weil es sehr weit von den niederdeutschen Dialekten entfernt ist.
Die Künstlichkeit klingt eben nicht natürlich, flüssig, weich. Tschechisch und Ivrit (Neuhebräisch) wurden beide künstlich reanimiert und da hört man die gleiche Künstlichkeit.
Was außer der Künstlichkeit macht den Klang des Hochdeutschen hart?
Aber die eingangs erwähnte Künstlichkeit ist der Hauptfaktor. Hochdeutsch entstammt der Schrift und nicht der Rede, oder mit Saussure gesprochen: … relève de la langue, et non pas de la parole.
Es kommt natürlich auch sehr auf den jeweiligen Sprecher an. Manche Leute sprechen halt sehr explosiv, andere ganz entspannt. Das macht sehr viel aus.