The rabbis taught that every person has two inclinations, the good and the evil. The evil inclination is also the source of desire and energy; a midrash says that without it no one would build a house or marry. The spiritual task is to harness it, loving God with both inclinations.
Yetzer ha-ra
The evil inclination: the human impulse toward selfishness and sin, balanced by the good inclination.
Also: evil inclination