My cousin developed lip-smacking. Why?

Some possibilities.. Lip smacking can be the result of a variety of different issues. It could nothing more than a behavioral habit, or it could represent an adaptive response to a new oral prosthetic device. It can also be a manifestation of neurological issues, such as dementia, parkinson's disease, or tardive dyskinesia. Especially if it is accompanied by other symptoms, it should be investigated by a doctor.
Dryness. Sometimes a dry mouth or lips can cause a person to constantly lick their lips. There are a number of things that can cause dry mouth or dry lips. It can be caused from from some diseases or various diferent kinds of medications.