In Lev. 19:18, why is it worded with רֵעַ, the word translated neighbor, rather than the word שָׁכֵן? Both occur throughout the books of the Old Testament and aren't isolated to a particular period of time. The common word in modern Hebrew for neighbor is שָׁכֵן. Not that common is רֵעַ. JPS1985 translates רֵעַ as countrymen in Lev. 19:18. The Septuagint (LXX) in Lev. 19:18 translates it with πλησίος (neighbor, near). The Latin Vulgate translates it with amicum (friend). The Hebrew lexicons say רֵעַ can mean friend, neighbor, countryman.
This is asking about the human side of the wording. Of course, for divine sovereignty, the uncertainty of the term רֵעַ leads to the lawyer to as the question who is my neighbor (probably רֵעַ) in Luke 10:19, but most definitely a question about Lev. 19:18. This question leads to the parable of the Good Samaritan. Note: lawyers hate ambiguity, but Jesus seemed to love it.