Question

Why do we say that charges are always produced in pairs?

Answer 1

Charges do not exist independently.
If, For eg, +ve charge is on the left side, then -ve charge should be on the left side. This means that if a neutral object has equal number of +ve and -ve charges and +ve is on the left side, then +ve is concentrated on the left side. So the -ve charge has to go to the right as it cannot be destroyed.