The lack of electron rich sources is due to the fact that these bonds are partially empty, even though they remain connected, since the region occupying the orbital is essentially dead. This electrophilic behavior is defined as empty space since everything inside is basically without any source of electricity except from outside the bond, since bonds tend to want to attract more to themselves(whether this be electric or non-electric can differ in most situations). The addition of the nucleophile means the continuous addition of a negative charge throughout the reaction, unless an electrophile also makes itself present to form a complete structure with no charge at all.