"America" is in the unique position of having no culture -- barring "make money" -- to assimilate to. This obviously leads to much greater ease of assimilation for immigrants; you don't even need to speak English. If you can achieve an upper middle class income, you have assimilated and are thus dubbed "American."

In Europe, it's much different. There's no "short cut" to being accepted in your host society; and I very much doubt anyone really assimilates into a European culture if they're not born within. There's a depth to it that is impossible to grasp unless you've been submerged in it for the entirety of your life.

You will never be a German. Should you decide to change your mind, and make the decision to move to and have children there -- then perhaps they will end up being German; but that path is closed to the individual you.

Accepting people is a long process when your standards are high; and short when low. Letting people come and go as they please does not make for a very stable society, filled with the types of persons that are beneficial to the whole.