In many cultures, the family is considered incomplete, and in danger of extinction, if their male line is not continued. Parents tend to look to a son to stay with them, care for them in their old age, and take responsibility for their funeral rites. This may be reinforced by certain religious beliefs and social expectations.
Within Hinduism, for example, it is a son's responsibility to nurture his parents in life, and their souls in death by lighting the funeral pyre that speeds their passage into the afterlife. So important is this, that women will ask for a Caesarian section if they know they are carrying their first son, rather than risk the baby with a normal birth. The Chinese attitude is that it is important to have boys, as only boys can carry on the family name. Similarly in Western cultures, families may go to great length to ensure continuation of the family name, if not the bloodline, which they see legitimately carried through a son.