@lakornkiki I think the others who actually read the book or understands Thai, can explain it better than me. But based on previous spoilers and comments, I believe that's just their fate in life. There's not A specific reason as to why their fate is that way, I think, it's just implied that it is that simply that way because "it's written in the stars" that way you know. Life is life, what is meant to happen will happen.
We'll have unanswered questions about this whole karma/fate thing, but I guess we're just supposed to think that that's the way life works. :/
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Kind of like I mentioned before about my parent's "experience" not sure on how true or accurate it is.
My parents were told, they've been spouses in their previous lives. My mom and dad both taking turning being the husband or wife each life. Their "karma" for not loving each other enough/faithfully in their previous lives. So until they can bore equal amount of son and daughters, their bond will never break and will continue this cycle until that happens. We currently have 5 girls, and 2 boys. My mom says, she'll deal with my dad another life time because she don't want deal with raising another bunch of us at her current age hahahahha