Women's competition
What would prosperous no had a family, it explicitly or invisibly will go a kind of competition among the female for the attention of the male part. Particularly acute the need of the daughter in the care of his father, and this same concern will certainly claim, and the girl's mother. In the conditions of modern life, when parents work late, and the head of the family, and all can be downloaded seven days a week, to give everyone the same amount of love becomes difficult. Therefore, there are cases in which the wife jealous of her husband to his own daughter, and this, in its turn, brings unpleasant imbalance in their relationship.
This can be attributed to General social and competition. For example, in the company of men women tend to attract the most attention and try to keep the popularity. This allows them to feel needed, desirable and in demand. While the mother may be a view that against the backdrop of his young daughter she would seem less interesting and beautiful. And thus, potentially deprives her daughter of attention and encroaching on demand.
This situation practically does not arise in the relationship with his sons. A woman does not feel in the boy of the opponent, rather it provides an additional "male" support. Son to mother is not a competitor but a source of love and possibilities.
The only man
In a situation if the woman was left alone with the child, the son may become a "substitute" non-existent husband. And there often is a psychological substitution when the whole of unrealized love is directed towards the boy. The woman ceases to feel the need for a life partner, but stops to look for him, beginning to concentrate exclusively on the son. To daughter, these feelings are impossible, since only in this case, the husband is replaced by the son, and the love for the boy transformered and enhanced.
Increase the attachment to the son of hope to help in the future. Thinking about old age or just the difficult moments of life, it is much easier to submit as the backbone of a successful son than a daughter. It is believed that a man is easier to ensure his mother's financial support, therefore, to rely on his logical.
Old traditions
For many centuries the way of life was such that the sons almost always had to live in the parental home, and the daughter of a mandatory order was issued to get married. Legalizing the relationship, the young woman moved to her husband and became a part of his family, forever leaving his native home. And so it happened that the sons perceived as something permanent and unchangeable, but the girls became rather a burden, as parents they had been using just before her marriage. Everything else for them had to prepare and give a dowry, which led to a not always feasible costs.
However, what would not exist regularities, mothers love their children regardless of gender, and each individual life story has its own characteristics.