I heard its what kittens do to their mothers when they are feeding from her, a way of stimulating the milk glands. So it is a memory of comfort. Dont know how true that is either?

Its hard to tell because cats also purr when they are in pain, not only when they are happy. Cats purr when in accidents like being run over etc & when they are giving birth also (and having had two labours & births myself, I know that its certainly not pain free lol)