The theory of reincarnation first appeared in India, a very long time ago, apparently even before the emergence of writing. But in European culture it became widespread thanks to hypnologists. Somewhere in the middle of the 20th century, hypnosis was fashionable, and the age regression method was quite popular
wikipedia.org Some hypnologist decided to go further. "send" a person at a time when he was not yet born, and then everything started to turn. :) The hypnotized began to speak in languages unknown to them before, name some dates and places, and even play musical instruments. True, such sessions at that time were sometimes, occasionally, held as a show ... but nevertheless, the idea went to the masses, incl. some research was carried out, similar to scientific research from afar. And only after that all sorts of Indian practices began to come into vogue, along with which the Indian explanation of these phenomena came.
I'll throw in a couple of theses:
The need to justify the world order. We want to believe that the world is just, reasonable, rational, that good is rewarded and evil is punished. And when we see that sometimes everything goes wrong, good people suffer, scoundrels thrive, and small children who have not yet done good or evil in their lives also suffer, get sick and die, this can destroy the picture of a just world. And then the concept of reincarnation comes to the rescue: reward or punishment in this life can overtake a person for the actions committed in the past.
Mystical experience. Since ancient times, different people in different parts of the world have experienced unusual states of consciousness. This was facilitated by: the use of certain plants and mushrooms; rhythmic dances around the fire to the sound of a drum; less fuss and information noise, allowing some to lead a more contemplative lifestyle; head injuries while hunting; other practices, both spontaneous and quite intentional. Among these altered states of consciousness, there was obviously an out-of-body experience, that is, the experience of travel outside the physical body. Yes, and our distant ancestors probably treated ordinary dreams as a form of such an out-of-body experience. All this led a person to think about the soul as a substance capable of existing outside the body, separately and independently of it. And why not assume that after the death of one body, this soul migrates to another?