The Law of Suprise, in the Witcher's world, is a tradition dictating that someone who saves another man's life has the right to demand, as a payment, something unknown to both persons. A typical request is "What you find at home yet don't expect", and often this would be a child born during the father's absence. This is detailed in the Witcher's Wiki and in this answer here on SF&F.
Some famous examples are those revolving about the birth and life of Ciri, or even the custom of Witchers who would use this tradition to acquire new boys to train them into new generations of mutant monster hunters.
I've read the books some time ago, so my memory is not so fresh, but in some episodes of the TV series first season, this tradition is considered something very important, almost having connotations of sacrality.
I wonder if it is ever explained (books, games or series) why such tradition is taken in such high account and how it originated, considered that often the price to pay is very high and maybe not something that the saved men would have consented to part if they could have chosen. Theoretically, this payment could even be something of low value, but we know that destiny, whatever force it could exactly be in the Witcher's world, is something very closely related to this *Law of Surprise", so this is unlikely.
Given these premises, I'm wondering why is the "Law of Surprise" regarded so high the Witcher's world? How it originated, and how it became a tradition so widespread and accepted by general consensus, when the price to pay would often be very high and demanding? Is this explained somewhere?