Why does His unity prevent people from answering Him?
Rashi: He knows their thoughts, and what they will say in front of Him, so answers are prepared in front of Him.
Ibn Ezra: Some say that the Beis is extra (it means, He is one). Really, it is not extra 1 . I cannot explain this, for there is a great secret in it.
Malbim: He already knew my path before I served Him, and that my Avodah will be refined like pure gold, and my foot will go only in His path. This forced me - I could not choose another path, for this would change His knowledge, which is Him Himself! His unity is absolute - it cannot increase or change.
E.g. Hashem uses one [Midah]. (PF)
Why did his Nefesh desire and do?
Rashi: Because it knew that no one can answer Him.
Malbim: It must desire and do as Hashem knows it will. Therefore, I do not deserve reward.
