By What Logic or Necessity?

Chapter I in Book 1 of Anselm's Why God Became Man raises the question upon which hangs the whole work.  The question is this: "By what logic or necessity did God become man, and by his death, as we believe and profess, restore life to the world, when he could have done this through the agency of some other person, angelic or human, or simply by willing it?"  Or, more briefly: "By what necessity or logic did God, almighty as he is, take upon himself the humble standing and weakness of human nature with a view to that nature's restoration?"