I really think it’s a cultural thing. It would be like me(an American) saying “how can I be more Japanese?”

To put a finer point on it, Americans have grown up in a culture where self expression and social interaction is encouraged. So we are less afraid of approaching a stranger for casual conversation. Add to that the fact that we, culturally, have very low self awareness. All we know is what offends other Americans. We rarely have the social intelligence to modify our behavior to match our surroundings.

So while, yes, Germans may be more reserved by nature; don’t think that is a value judgement. If you want to be more outgoing do it in a German way. Americans are the way we are because we are Americans.

That said, if I could offer a suggestion to mitigate your social anxiety; I would suggest that other people don’t think about you and your behavior anywhere near as much as you think they do. Most people don’t give a shit. If you can’t think of anything to say but you want to start a conversation a compliment is always a winning strategy. But nothing is a substitute for genuine curiosity.