One part of it, at least for me, is the psychadelic "converging on rebirth" feeling. It's a very geometric type of high, most likely due to the NMDA agonism, induced dissociation / depersonalization. Much shorter duration than LSD, and less chance of bad thoughts/actions since you are pretty much sedated and unable to panic. Ketamine is really therepeutic mentally.
The doses given therapeutically are lower than recreational ones.
That's not true. I've done ketamine recreationally and also through an MD via infusion. The infusion had me in a k-hole for an hour or two. Recreationally, I've been in a k-hole for maybe 15-30 minutes.
Others here in this thread agree that therapeutic doses are smaller, and after a brief search it seems papers do too.
Perhaps your effects were down to the ROA?
Not fully. One part is pretty persistent NMDA receptor blockade. How does that tie into depression if at all, we do not know.