One has start trusting the other in order to earn the mutual trust. It is known that the US uses spies to collect information from different countries. The paranoid US does not trust any country that can cause potential harm to her. Germany should trust and cooperate with the US if they have nothing to hide from the US. When they are sharing every information, spying would be pointless.
Eventually, any breach of trust can be forgiven - providing that the breach doesn't continue. The key to moving on is admitting fault and then ensuring it isn't repeated. If we need information from Germany this needs to come through diplomatic channels, as it always should have done. It's only a shame this isn't how it's been done so far.
While the US-Germany spy scandal is embarrassing in some respects it should not effect the long term relations between each nation. The US and Germany have been key allies since WW2 this friendship being the cornerstone of the defence of Europe in the Soviet era, Allies spy on each other regularly at international conferences and most politicians know that, The future defence of both countries depend on mutual trust.
It is as simple as it gets, Germany trusted the US and US betrayed them. It can be observed through this very spy scandal that was uncovered recently. What we all do not know is that there may be more than just spies in Germany's territory and that they haven't uncovered these yet so it's obvious that Germany cannot trust the US and that it should take precautionary steps in order to avoid such incidents in the future.
While I'm fine with the US being allies with Germany and working with the country with NATO and the United Nations, it would be very foolish of us to ever trust Germany. I'm a firm believer in history repeating itself, and considering Germany started World War One and Two, we need to make sure Germany never has the opportunity to start a new global war in the future.