The responsibility of being the only son in the family is a
socially induced phenomenon. (im going to post my own opinions now
without even doing any research so if anybody would care to correct
me or back me up, i welcome it)
Since the time when mankind discovered that the sperm that comes
from men will impregnate the woman who has the egg, it has been
thought that a family's "line" will be carried on by the male. This
ideology is not only present in oriental civilizations but in
western civilizations as well.
The responsibility of the male in a family was to spread his seed
and therefore his family line so that the family will grow
"stronger". As numerical superiority was seen as the most basic
measure of strength, the more descendents in a family tree, the
stronger a family is. Such responsibilites are extended to the
security and the well being of this "family" which probably gave
rise to how many societies place breadwinning responsibilites on
the males.
In present day, such "responsibilities" are slowly diminishing in
both western and oriental but comparatively, it is still more
prevalant in oriental ones. The more traditional generation prefers
to have a boy in the next line of descendents not only to keep the
family line going, but to keep the family "strong". Many rituals in
oriental customs still place importance on the eldest male in the
family, the most notable of all being chinese funerals.
Thus being the only "son" in the family carries all the burden that
could have been shared by brothers that he doesn't have.