It is impossible to achieve true upward movement without some kind of opposition. Just as air pressure creates a stronger upward movement than that which could be obtained by flotation, so too, for a person to transcend the materialism of the world and achieve a heightened closeness to G‑d, it is not enough to involve the good inclination alone. It is specifically through the opposition of the evil inclination, and the extra drive one exerts to overcome it, that one attains a more powerful “lift,” and hence greater heights.1

Igros Kodesh, Vol. 30, p. 224
Translation taken from Mind Over Matter