We have to love our neighbor because he is there.

We make our friends; we make our enemies; but God makes our next-door neighbor. ... That is why the old religions and the old scriptural language showed so sharp a wisdom when they spoke, not of one's duty towards humanity, but one's duty towards one's neighbor. The duty towards humanity may often take the form of some choice which is personal or even pleasurable. ... But we have to love our neighbor because he is there — a much more alarming reason for a much more serious operation. He is the sample of humanity which is actually given us.

G.K. Chesterton, Heretics,
On Certain Modern Writers and the Institution of the Family

I have seen the first line of quote interpreted, often by realtors, as meaning that our neighbors are a precious gift. And they are, but not in the sweetly sentimental way that the realtors put forward. We may, in fact, like our neighbors. But often our neighbors are a source of great trial. They are given to us by God in order to try us, to test us, to teach us.

What is equally sobering is we are given to them, as their neighbors, for the very same reason.