"Men and angels belong to separate categories of existence.
Even if we ignore the human body and look only at our apparently more angelic aspect, the soul, the differences are great.
The human soul is a heterogeneous, complex entity composed of distinct elements, whereas an angel is homogeneous, a single essence, and thus ultimately unidimensional.
Furthermore, the human being, by virtue of the multiplicity of facets in his personality, with the implicit capacity for internal contradictions and conflicts, and by virtue of his soul, which contains a spark of the divine, possesses the power of discrimination, in particular between good and evil.
As a consequence, man had the potential to reach great heights, and also to fall to abysmal depths.
Not so the angels, which are always the same.
Whether an angel is ephemeral or eternal, it is static and remains fixed in the coordinates of content and degree in which it was created."
From the essay "Worlds, Angels, and Men" by Rabbi Adin Steinsaltz