Well, actually it i no big question. Your answer isn't entirely correct, because it is not a simple "switching off" it is also a matter of "switching on" or overexciting. But that is not what i am looking for. I realize that you are no expert on this, therefore i am looking for a simpler answer.
Let me try to ask differently: What do you think is the reason for substances that are not naturally produced in the body to have such an (profound) effect on the body?
(Please abstain from discussing the meaning of the word "naturally", if you can help it. I know you know what i am talking about.)
I don't think I know an answer. All I can think of quickly (and I don't think it's a good answer at all) is that life is like one of those video games where you have to find magic trinkets that give you special powers and/or allow you to advance to the next level. But don't ingest too much of the magic poppies or you'll go nuts and fail your mission. We are not told the rules before the mission. We have to discover them for ourselves. Perhaps by trial and error.
I doubt this is what you are after since you are hoping for a simple 'answer'. You will probably think that I'm pretending to not understand you but I promise you that I am trying to understand your question.
Wait. I may have thought of something better while I've been typing. A human partner is found outside the body and can have a profound effect. So can a sunset. A painting. A panoramic view. A book. A movie. Chocolate. So lots of things from outside the body can have a profound effect. Now why do they have a profound effect ?
Here's my theory - I think that the human self is severely lacking and is forever looking for something to complement/supplement himself. He will try anything and everything with varying degrees of success - marriage, kids, sex, drugs, food, music, thrills, knowledge, money, power, sport, talking, thinking, religion, prayer, meditation...
And all these things can give enjoyment and fulfilment to varying degrees.
Many of these things are outside and the individual hopes that he can somehow make himself 'whole' through them. Sometimes he may also start to look inside for 'answers' eg meditation.
The NDE and the drug experience can both also give a sense of wholeness albeit temporarily. Other items on the list above can also.
This does seem to be forming in my head into a fairly cohesive theory because it links back to the sense of unity which is occasionally experienced (usually rare) as opposed to our normal life made up of lots of discrete things ie separateness.
This separateness then links to the sense of lack within the self - which is the reason why so many people are searching for something which will make them whole again. Some Eastern philosophies might describe the solution as the Nirvana or Unity experience whereby the separate self can actually realise that it is already
united with the whole and that realisation can be immensely joyful.