To be sure, power over people is an expression of superior strength in a purely material sense. If I have the power over another person to kill him, I am "stronger" than he is. But in a psychological sense, the lust for power is not rooted in strength but in weakness. It is the expression of the inability of the individual self to stand alone and live. It is the desperate attempt to gain secondary strength where genuine strength is lacking.
The word "power" has a twofold meaning. One is the possession of power over somebody, the ability to dominate him; the other meaning is the possession of power to do something, to be able, to be potent. The latter meaning has nothing to do with domination; it expresses mastery in the sense of ability. If we speak of powerlessness we have this meaning in mind; we do not think of a person who is not able to dominate other, but of a person who is not able to do what he wants. Thus power can mean one of two things, domination or potency. Far from being identical, these two qualities are mutually exclusive. Impotence, using the term not only with regard to the sexual sphere but to all spheres of human potentialities, results in the sadistic striving for domination; to the extent to which an individual is potent, that is, able to realize his potentialities on the basis of freedom and integrity of his self, he does not need to dominate and is lacking the lust for power. Power, in the sense of domination, is the perversion of potency, just as sexual sadism is the perversion of sexual love.
excerpt from: Erich Fromm (1941) Escape from Freedom.
pdf: Craving for Power