The twins struggled in the womb, on the way out of the womb, and would continue their struggle.
God told Rebekah that the younger son would rule over the elder--a reversal of the expected. Primogeniture served as a way to preserve the family's holdings without war breaking out within the family.
Isaac preferred Esau, the son who could hunt; Rebekah preferred Jacob, the one who stayed at home. Or, Rebekah had been given advance information by God that Jacob was to be the stronger one, the one who was to be served by his elder brother, and Isaac had not been given this information.
Three ways to interpret this reversal:
God can make an expected choice
One who has been designated the winner can voluntarily give up his reward
Reminder--your brother may want to change the rules
Esau gives up his birthright because he is famished. Why does his brother make this demand?
OTOH: If the Lord intended for Esau to serve Jacob, why do we say that Jacob tricked his brother out of his birthright?
From reading Come Out, My People! by Wes Howard-Brook:
Jacob will live not only to regret this "victory," but will desperately seek to "return" what he has taken. Both the taking and return are part of Genesis's parody on success-seeking apart from obedience to the Creator. Jacob has as yet no direct experience of YHWH. His cleverness makes a fool and an enemy out of his brother and the consequences chase him for decades.