Messed up neurotransmitters happen. When serotonin, GABA, endorphins, dopamin, etc., are lacking (and there are tests to measure them), problems ensue. And they alter moods...which sometimes results in genuine (meaning not self-pity or spiritually induced) depression.
When sin entered this world, it brought about immediate spiritual death. But another thing it did was bring about physical malfunctions...and those malfunctions continue to this day. Mutations, imbalances in different areas of the body, genetic disorders, etc. are all real. And a result of sin. NOT personal sin, but that sin that entered the world when Adam disobeyed God. Our bodies are corrupt and they betray us, whether there is a test to "prove" a malfunction or not.
I would agree that psychology is not the answer (just an FYI - the actual FIRST meaning of "psyche" was "breath," later changed to "soul," and even later "mind" added to it...probably because most involved in the field don't want to admit people have souls...). But depression is not limited to the psychological realm, in diagnosis or treatment.
And let's be really, really careful to avoid making fellow Christians feel like they have a spiritual problem because we don't understand their physical issues - either by intent or by poor choice of words.