When abroad, you become accutely aware of other Americans. If you live abroad, you try to disassociate yourself with them in the place you live; they are tourists to look down at, while you have clearly adapted and should be considered local. But once you travel from your abroad location, you are again a travelling American, and without even trying end up talking to other Americans in airports, trains, and sites. It's helpful that during spring break all the abroad Americans seem to be travelling around, so there is a sharp increase in 20-some-odd year old Americans galavanting around Europe.
The real point of this theory is that upon your return to the US, you feel surrounded. After spending 5 months avoiding being associated and tuning your skills to identifying other Americans, you return to a place where most people are what you've been trying to avoid. For the first few days back, you're going to feel completely out of place. Not culture shock, mind you, but just a heightened awareness that everyone around you is American.*
*This is not a commentary on American habits or anything else.
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Thanks for reading,