Dallas Willard in The Scandal of the Kingdom:

The question, “Who is my neighbor?” is the wrong question because there’s no end to the way it can be debated and discussed. The kingdom question is, “To whom will you be a neighbor?”

To “love your neighbor as yourself” is not a matter of identifying and making a list of neighbors and then making it a project to go around and love those people. It’s an active love, where we are alert and aware because the next person we come across— though we may have never seen them before and though they are very unlike us—may turn out to be someone to whom we will choose to be a neighbor. This places the emphasis on neigh-bor-ing. Neighboring is an opportunity in the kingdom of God.