Why is ketamine so effective at treating depression?

It's an NDMA receptor antagonist, which means it inhibits the production of glutamate neurotransmitters.

Could it be that glutamate plays a role in establishing painful memory pathways that lead to depression?

You would think depression has everything to do with serotonin. But ketamine doesn't affect serotonin at all. How much do we even know about how all these things work?