This my personal theory, which I dubbed “The Bad Green Milk Theory” which I posted originally on Reddit. Here’s a bitlink: http://bit.ly/badgreenmilk
The Thala-siren Green Milk Contained Parasitic Microorganisms That Altered Luke's State of Being
I mean, far-fetched for sure, but this explains a lot actually, namely Luke’s seemingly out-of-character behavior.
“Parasitic Microorganism – Vector-transmitted parasites rely on a third party, an intermediate host, where the parasite does not reproduce sexually to carry them from one definitive host to another. These parasites are microorganisms, namely protozoa, bacteria, or viruses, often intracellular pathogens (causing disease).”
The milk produced by the female Thala-siren contained a viral parasite that accumulated in the host’s brain and kept them from wanting to leave the island. The longer it was ingested, the more the host was adamant about living on the island and hating outsiders. Additionally, the parasites would soon be able to alter brain chemistry enough to control the host’s thoughts and actions.
Wookieepedia’s entry concerning this milk states that it was “nutritious” and that makes sense because the parasites would want their hosts to remain healthy.
“The females, which were known as sea sows, produced a nutritious green milk, which was harvested by Luke Skywalker and the native Lanai.”
Perhaps whatever Dark Side entity lived underneath the island (that Rey was drawn into the cave by) had a hand in this.
The parasite was a siren song of sorts, passed into the host by the green milk, separating them from their normal self. Only when Force Ghost Yoda showed up was Luke able to overcome the parasites and break free.
Luke: I was weak. Unwise.
Yoda: Lost Ben Solo, you did. Lose Rey, we must not.
Luke: I can’t be what she needs me to be.
Yoda: Heeded my words not, did you? Pass on what you have learned. Strength, mastery. But weakness, folly, failure also. Yes, failure most of all. The greatest teacher, failure is. Luke, we are what they grow beyond. That is the true burden of all masters.
This is the moment Luke comes back to his senses and takes action, if only for a few fleeting moments before his demise.