Question

why do we say that charges are always produced in pairs

Answer 1

Simply because, everything is neutral in the world. There is nothing to have absolute charge over it. It is us who creates charge from neutral objects. Therefore when we create charge by any mean from neutral object, charge in pair will only be produced. ETRICAL

Answer 2

bcozz if a body gives charge to another body the gaining body acquire -ve charge and losing body acquire +ve charge hence both charges are produced.