Christ willed to be born in Bethlehem for two reasons. First, because "He was made . . . of the seed of David according to the flesh," as it is written (Romans 1:3); to whom also was a special promise made concerning Christ; according to 2 Samuel 23:1: "The man to whom it was appointed concerning the Christ of the God of Jacob . . . said." Therefore He willed to be born at Bethlehem, where David was born, in order that by the very birthplace the promise made to David might be shown to be fulfilled. The Evangelist points this out by saying: "Because He was of the house and of the family of David." Secondly, because, as Gregory says (Hom. viii in Evang.): "Bethlehem is interpreted 'the house of bread.' It is Christ Himself who said, 'I am the living Bread which came down from heaven.'"
Thomas Aquinas, Summa Theologiae 3.35.7.