Why ask: What is a citizen?

I find myself now on the brink of dual citizenship. I will retain my natural citizenship (US) and apply for (and hopefully gain) another citizenship in Canada. I have made the pragmatic decision to go forward, but doing so did give me pause. Most people wonder how I could possibly hesitate. I want to work through just why I did pause, why I went forward anyway, and just what it does mean to be a citizen of a nation in an era marked by human migrations and globalization.