Cats purr when satisfied, in pain, giving birth and dying- indicating that they purr for a variety of reasons, whether they are stressed or happy.
Kittens that purr during nursing appear to be signaling that they are safe and healthy to their mother. Likewise, the mother may purr during nursing to help guide her kittens; it is also thought that purring is an essential part of mother-kitten bonding.
Research at the University of Sussex showed that cats are capable of masking a high-pitched cry within their purr that nears the frequency exhibited by the cries of human infants. This unique purr appears to be linked to the cat's desire to eat and is used to grab the attention of its human caretakers.