The ability to engage in holy endeavors–Torah and mitzvot-is itself the ability to be completely opaque to any holiness.
An angel sees God but cannot perform the slightest mitzvah.
Man, who cannot see God, is capable of performing the mitzvot.
This is the essence of a human being.
His power to choose–his ability either to cleave to God or to deny His existence–creates the human psyche.
–Rabbi Adin Steinsaltz
From Understanding the Tanya, Chapter 35, by Rabbi Adin Steinsaltz