To need to do something is a class of motivation that belongs to an urge that comes of abundance.
An agent divests of waste out of need to satisfy an urge. The obligation that comes with accepting a gift is what one feels because of the need the gift established in the person. We need to earn and we need to deserve and we need to act for the sake of achieving justice-as-deservedness. When we are in an undeserved state our sense of justice sets up a need in us that registers as an urge to perform a duty to return ourselves to a state of deservedness. A need is a discomfitted feeling which the satisfaction of some urge would allay.
Needs and urges are sensational, hormonal counterpoints to rational incentives and preferences.
Urges give rise to urgency. Needs and urges have a temporal aspect to them that incentives and preferences lack.