A typical response by those who advocate closed (or, at least, tightly controlled) borders is this:
“Do you leave your front door unlocked?”
There are many things wrong with this statement, and people far smarter than I have dealt with it. But I have two main problems with it:
1) It assumes the government owns the country. I reject that as a libertarian and a moralist.
2) It’s an inapt analogy. I do lock my door to prevent someone from stealing what little stuff I own. That is because a good person would have no cause to enter my apartment unless I invite them in. Conversely, a good person would have many reasons to cross the border from Mexico to the US (or anywhere else). I can reasonably assume that a person who crosses my threshold wishes to cause me harm because he was not invited in. One cannot reasonably make the same assumption for national borders.