Nervousness and stress comes to mind first but I feel there is more to it than we can chew on in one bite. Perhaps the body craves for a particular 'material'. In many cultures pregnant women have strange cravings. People suffering from diabetes have a craving for sweets. I had read about a south American child who would chew off anything - pencils, chairs, desks etc.
Later it was found that he had a medical problem.
A child in our family has a special liking for plastics - yes even the TV Remote device has been chewed off!
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