I'm not really sure how or why people tend to consider Germans as being so serious?
Is it because we tend to be more formal when meeting people or conducting business?
Is it our lack of familiarity with someone we just met or the unwillingness to say "come by sometime" especially when we don't really mean it??
Xenophobe's Guide to Germans, a satirical guide to understanding the quirks and traits of Germans, states, "Germans take their humor very seriously ... It is no laughing matter."
There are many stereotypes of Germans: they love soccer, beer, and sausage ... they are direct and blunt in speaking ... they are punctual and organized. And on it goes. I only need to show you a picture of a Bavarian in his traditional Lederhosen, and most people would correctly recognize that person as "German". However, as is the case with most stereotypes, it is not always consistent. Take the Lederhosen-clad Bavarian; he only represents one particular state (Bundesland) in Germany, not all German region's traditional dress. Consider also that I am German-born, but I am almost always running LATE! Now, I must defend my German-ness by noting that I *am* neat and organized.
I haven't taken any formal surveys, so my facts are just observations, but maybe Germans are seen as serious because they are so direct (in general), especially in business dealings. It is what it is, you know. They can be viewed as rude, arrogant, or maybe even boring. Also, Germans, as a whole, are not known to be "smiley" people. They usually only smile if they are sincere. In America, we seem to interpret smiling as being friendly and open. In fact, at the organization I currently work for here in Arkansas, there is an implemented 10/5 rule: "when walking down a hallway, you are to SMILE at any approaching person at 10 feet and greet them at 5 feet." In Germany, you might only smile and greet a person or co-worker if you know them personally. Otherwise, just passing each other without making eye contact in a hallway is not considered rude at all.
I think German society is slowly adapting some of the "American" ways, perhaps through the influence of the military presence in Germany for so long or maybe because of all the American movies and TV shows that are shown in Germany. But I remember a day when my older German relatives would almost mock the American's practise of saying "Have a nice day" to every customer and stranger they meet or asking a stranger, "How are you?" (as if they really cared!?!). While it IS German etiquette to say a friendly "Guten Tag" (or "Gruss Gott" or whatever greeting is normal for the region) when you enter a shop, particularly a small one, it has NOT been the practise to say "Have a nice day." However, the last time I was in Germany in 2012, we were wished, "Schönen Tag noch!" (= <have a> good day now or also) by a store clerk several times.
Despite all the reputed seriousness, Germans CAN have a good time and share a great joke! My Opa and other relatives of mine were great story tellers, and often, a funny story could have us all in stitches. And there are plenty of German comedians and comedic film stars, stemming back to Heinz Rühmann, who starred in a famous movie Der Hauptmann von Köpenick ("The Captain of Köpernick"), based on a true story of a German imposter who posed as a Prussian military officer and gained sudden and great respect (filmed in 1956 and set in 1906).
Another famous German comedian my family enjoyed is Hans Moser. One of his most famous movies that I remember is Hallo, Dienstman ("Hello, Porter") made in 1952. You can read about it at IMDb's website. It is interesting to note that Charlie Chaplin bought the rights for the sketch this movie is based on. He never performed it though, because he felt he couldn't keep up with Hans Moser's wit.
And consider that every New Years Eve since 1963, an 18 minute comedic sketch is shown without fail in German television, Dinner for One. The introduction of the clip below is in German, but the actual story is in English, starting at the 2:25 mark.
A fond memory of my childhood was watching the German TV variety show, Verstehen Sie Spass? ("Do You Understand Fun?"). The show has run in Germany since 1980. It is equivalent to America's Candid Camera.
Germans aren't really as serious as they are reputed to be! Just get to know one, and you'll soon find yourself smiling and laughing.