Luther and why it was the Son who became incarnate
Then on the question as to why the Son, in particular, became incarnate, Luther makes the suggestion that it was "fitting" for him to do so because he was first born in eternity and then "was physically born and became a son."
Kevin Giles, The Eternal Generation of the Son: Maintaining Orthodoxy in Trinitarian Theology (Downers Grove, IL: InterVarsity Press, 2012), 5