Greek Expositor (anonymous) — as excerpted in the Catena Aurea on Luke 2:1-5

“(Irenæus cont. Hær. 1. 3. c. 11.) Now he added, a city of David, that he might declare that the promise made by God to David, namely, that from the fruit of his loins there should go before him a king for ever, (2 Sam. 7:12.) was already fulfilled. Whence it follows, Because he was of the house and lineage of David. (Ps. 132:11.) But since Joseph was of the family of David, it pleased the Evangelist to make known also that the Virgin herself was of the same family, because the Divine law enjoined marriages between those of the same line; and therefore it follows, With Mary his espoused wife.”