In material matters, one who is "satisfied with his lot" (Ethics of the Fathers 4:1) is an individual of the highest quality; a person possessing this trait will attain the highest levels. In spiritual matters, however, to be satisfied with one's lot is the worst deficiency, and leads, G-d forbid, to descent and falling...
Rabbi Schneur Zalman of Liadi (quoted in Hayom Yom, Sivan 30)