I got into a conversation recently with a friend who brought up this exact issue: the US has been built by immigration from the beginning, so why is it scary to us? I didn't come up with a good reason then, but thinking about it now, it seems that each wave of immigrants is more and more different from the white mostly-English mostly-Protestant men who set up the original structures of this country -- different races, different cultures, different religions. Might that be part of why the reaction to immigration has gotten no better over time, and in some ways maybe worse?