Human relations tend to be more difficult when you're dealing with someone who weighs 30 kilograms more than you do. That's when you worry about whether a well-meaning gesture could produce complications. We have no problems with countries like Madagascar or Bolivia, for example. But Germany is our neighbor and we have a shared past. Besides, Germany is powerful and ambitious and more than four times as large as we are. It makes complete sense that we would act cautiously. It's simply Realpolitik.