Selective breeding, if I had to guess.
Say you find two stray cats: one that does not like being petted, and one who does. You will more likely treat the friendliest cat better, providing it more food and shelter and increase its chances of surviving compared to the “unfriendly”. Multiply this for thousands (?) of years and you will get a majority of animals that enjoy being petted since the other ones are less likely to thrive.
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