Ay yi yi. Delve into the expat blogosphere or chat forum and you'll find people lamenting how cold Germans are. Americans especially are wont to say how their little feelingsies were hurt when the meanie German didn't return their goofy grin. Others have felt the cold, hard stare of a German having a good look. Or heavens forbid one needs customer service.
Do those reporting on this try to learn German and try getting to know Germans personally, in a social setting, instead of out in public? Have they ever tried to learn about a culture and find out what facial and body expressions are typical for that area? Instead of expecting the store clerk, who is there to serve all the other waiting customers, to be a perfect test person for trying out German, have they considered finding an actual tandem partner to work on the language in a non-stressful environment?
I don't find Germans cold at all. Do I find the German culture more serious? Yes, I do, but that doesn't mean that its residents are without mirth. Do I find their behavior with strangers a bit more reserved? Yep, but I appreciate this in some ways.
When I'm out running errands, I have no desire to smile at random strangers or chitchat with the cashier at the store; I'm going about my business, and to be frank, I'm not interested in what others are doing. As an introvert who is socially quite active and plans many group events, I feel as if I don't get enough time to myself, so errands are "me time" and allow for introspection. I don't mind a polite hello-your total is-thank you-have a nice weekend-goodbye with the store cashier; that is sufficient conversation with a stranger.
Why should there be a pressing need to look at/smile at strangers on the street (other than to have a general awareness of your surroundings for safety reasons)? I find it very curious to hear from Americans who feel slighted if random people on the street don't want to engage in smiling at one another or saying hello. I try to avoid doing this with strangers unless it would be blatantly impolite to do so in a situation with low risk factors. For me, it's a safety measure and means of avoiding harassment. I've found, from attending university somewhere that has a tough downtown scene, that not allowing strangers to engage you or approach you is part of keeping yourself safe. While I don't feel that it's so insecure here in Germany, it's still important to be careful.
Before judging random Germans to be cold (or any other culture, for that matter) just because they don't want to engage in your ways of interacting, I would like to suggest integrating more into German life. Meet people in social situations where they're more likely to have the time and interest to meet others. Don't judge things based on some very superficial interactions.