I had just read something on this but I can't find the source now but basically going from my memory it has to do with the five shens: hun, po, shen, yi, zhi, and in paricular it is the hun which resides in the liver becomes connected to the other person and, in a sense, part of it is wandering or away from the body which affects all the 5shen mainly disrupting the yi/spleen which causes the loss of appetite, which in turn affects the zhi/kidneys and you lose the willpower to want to do anything. It's a roughly 3 day ordeal until everything rebalances. There also is something to do with 欲 'yu' desire- the character is made up of a 谷 'gu' (a ravine, gorge, hole) next to 欠 'qian' (lacking, deficient) meaning a 'hole that needs filled'. The sickness is significantly worse after you've been with someone for a time and then break up, the hole may take years to fill and in some cases may always be there.
The hun being to drawn another person in the first place is because it comes from the heavens and is following the path that adheres to 靈 'ling' the spiritual world or in a sense the destiny that's been set out for you. Your xin shen/ heart mind (you, consciousness) can choose to follow the hun's desire or go against it.
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