112 All these things, however, Venerable Brethren, depend in large measure on the due preparation remote and proximate, of the parties for marriage. For it cannot be denied that the basis of a happy wedlock, and the ruin of an unhappy one, is prepared and set in the souls of boys and girls during the period of childhood and adolescence. There is danger that those who before marriage sought in all things what is theirs, who indulged even their impure desires, will be in the married state what they were before, that they will reap that which they have sown; indeed, within the home there will be sadness, lamentation, mutual contempt, strifes, estrangements, weariness of common life, and, worst of all, such parties will find themselves left alone with their own unconquered passions.
Source: Casti Connubii (Vatican.va)