All the mass we ever encounter is positive - nothing weighs less than zero. What makes us think charge is different that there are both positive AND negative charges?

Based on the number of positive and negative charges

Explanation:

In a positive charge particle, the number of proton is higher than the number of electron. Due to this fact alone, we can come to the conclusion that positive charge consists of more number of protons than electrons. In a negative charge particle, the number of electron is higher than the number of proton. Due to this fact alone, we can come to the conclusion that negative charge consists of more number of electrons than protons.