Nucleophile is a species that should have

Nucleophile means that which attracts nucleus. Nucleus is positively charged as it contains the neutrons( which are neutral) and protons which are positive in nature. Opposite charges attract. So to attract positively charged nucleus the nucleophile should be negatively charged. Negative charge is borne by electrons so nucleophiles should be electron rich species i.e., able to donate one or more pairs of electrons.