"The middle period of life is a time of enormous psychological importance. With progressive maturation he widens his horizons and his own sphere of influence, hopes and intentions, all directed to extending the scope of personal power and possessions; desire reaches out to the world in ever widening range. The will of the individual becomes more and more identical with the natural goals pursued by unconscious motivations. Thus man breathes his own life into things until finally they begin to live of themselves, and to multiply, and imperceptively he is overgrown by them...
Midlife is the moment of greatest unfolding when a man still gives himself to his work with his whole strength and his whole will. But in this very moment, evening is born, and the second half of life begins. Passion now changes her face and is called duty; 'I want' becomes the inexorable 'I must', and the turnings of the pathway that once brought surprising discovery become dulled by custom. The wine has fermented and begins to settle and clear. Conservative tendencies develop if all goes well, instead of looking forward one looks backwards, most of the time involuntarily up to this point. The real motivations are sought and real discoveries are made. The critical survey of himself and his fate enables a man to recognize his peculiarities. But these insights do not come to him easily. They are gained only through the severest shocks.
Since the aims of the second half of life are different from those of the first, to linger too long in the youthful attitude produces a division of the will. Consciousness still presses forward in obedience, as it were, to its own inertia, but the unconscious lags behind because the strength and inner resolve needed for further expansion have been sapped. This disunity with oneself begets discontent and since one is not conscious of the real state of things one generally projects the reasons for it upon one's partner. A critical atmosphere thus develops the necessary prelude to conscious realization."
Source - Carl Jung