The potential energy increases because the charges will repulse each other. Since the electron has a negative charge, and you are bringing two of them together, the two negative charges will repel each other. Using Coulomb's law, F=kq1q2r2 , we can find the potential energy of the system. Since q1 and q2 are both negative, F must be positive. As r decreases, F increases, which means that the potential energy increases.
