(Latin: movere, to move)
Whatever moves the will to act, necessarily something good, or something that attracts the mind under the appearance of good, useful, or pleasant.
Every voluntary act must have something good, or apparently good, in view.
Happiness, immediate or remote, is the motive of every action, the happiness one derives from life, health, home life, friendship, riches, distinction, pleasure, uprightness, knowledge of God in the present, and the happiness of perfect union with God in the future.
Motive thus becomes a principle of action on account of the objective worth of the good to be obtained and of the subjective disposition or inclination of the agent.
This is why we are said to know a person from motives, to judge of character by the aims and ideals of the person in question and of the methods employed to attain them.
New Catholic Dictionary