Humanity's animal instincts lead it to divide others into "them" and "us", not necessarily with any meaningful boundary. This may have been useful habit at one stage of our evolution, but now we need to rise above it (and some other instinctive behaviour) through conscious choice. Patriotism/nationalism harnesses this divisive instinct on a national scale, but at its worst is just collective selfishness.

Incidentally, patriotism may be defined as "love of one's country", but in this context what do we mean by "love"? What do we mean by "country"? I take "love" to be that which leads us to prefer another's advantage to our own. And "country" is, for me at least and in this case, not the geographical area where I happen to have been born and now live, but the political entity of which I am legally a citizen and upon whose actions I have some slight influence through voice and vote.