Explanations for depopulation in the first world generally depend on strictly material observations of the population in question. For instance, economists point toward the rise of the Welfare State because it replaces the incentive to have children (economic security) with institutional guarantees sustained by taxation of the wealth created by others. But there are also psychological causes.
From Decline of the West :
And then, when Being is sufficiently uprooted and Waking Being sufficiently strained, there suddenly surges into the bright light of history a phenomenon that has long been preparing itself underground and now steps forward to make an end to the drama - the sterility of civilized man. This is not something that can be grasped as a plain matter of Causality; it is to be understood as an essentially metaphysical turn towards death. The last man of the world-city no longer wants to live - he may cling to life as an individual, but as a type, as an aggregate, no, for it is a characteristic of this collective existence that it eliminates the terror of death. That which strikes the true peasant with a deep and inexplicable fear, the notion that the family and name may be extinguished, has now lost its meaning. The continuance of the blood relation in the visible world is no longer a duty of blood, and the destiny of being the last of the line is no longer felt as a doom. Children do not happen, not because children have become impossible, but principally because intelligence at the peak of intensity can no longer find any reason for its existence.
Let the reader try to merge himself in the soul of the peasant. He has sat on his glebe from primeval times, or has fastened his clutch in it, to adhere to it with his blood. He is rooted in it as the descendant of his forbears and as the forbear of future descendants. His house, his property, means, here, not the temporary connexion of person and thing for a brief span of years, but an enduring and inward union of eternal land and eternal blood. It is only from this mystical conviction of settlement that the great epochs of the cycle pro-creation, birth, and death - derive that metaphysical element of wonder which condenses in the symbolism of custom and religion that all landbound people possess. For the "last men" all this is past and gone. Intelligence and sterility are allied in old families, old peoples, and old Cultures, not merely because in each microcosm the overstrained and fettered animal element is eating up the plant element, but also because the waking-consciousness assumes that being is normally regulated by causality. When the ordinary thought of a highly cultivated people begins to regard "having children" as a question of pro's and con's, the great turning-point has come. For Nature knows nothing of pro and' con...
...When reasons have to be put forward at all in a question of life, life itself has become questionable. At that point begins prudent limitation of the number of births. In the Classical world the practice was deplored by Polybius as the ruin of Greece, and yet even at his date it had long been established in the great cities; in subsequent Roman times it became appallingly general. At first explained by the economic misery of the times, very soon it ceased to explain itself at all.
In our own time the whole of Greece has been subject to a low birth-rate and a general decrease of the population, owing to which cities have become deserted and the land has ceased to yield fruit, although there have neither been continuous wars nor epidemics. 6 If, then, any p385one had advised us to send and ask the gods about this, and find out what we ought to say or do, to increase in number and make our cities more populous, would it not seem absurd, the cause of the evil being evident and the remedy being in our own hands? º 7 For as men had fallen into such a state of pretentiousness, avarice, and indolence that they did not wish to marry, or if they married to rear the children born to them, or at most as a rule but one or two of them, so as to leave these in affluence and bring them up to waste their substance, the evil rapidly and insensibly grew. 8 For in cases where of one or two children the one was carried off by war and the other by sickness, it is evident that the houses must have been left unoccupied, and as in the case of swarms of bees, so by small degrees cities became resourceless and feeble. 9 About this it was of no use at all to ask the gods to suggest a means of deliverance from such an evil. 10 For any ordinary man will tell you that the most effectual cure had to be men's own action, in either striving after other objects, or if not, in passing laws making it compulsory to rear children. Neither prophets nor magic were here of any service, 11 and the same holds good for all particulars.