Taking Jacob’s bargain demonstrates this. Jacob was, in essence, issuing a test for his brother, one that Esau failed. Had he stood his ground, had he fought for the birthright and taken up his responsibilities as the first-born son, Jacob wouldn’t have had to.
Esau views being first born as an excuse to do whatever he wants while Jacob embraces the responsibilities. And there’s the responsibility that has yet to be talked about, too: Rebekah.
The first-born son inherits when Isaac passes on. Rebekah cannot inherit (I didn’t make the rules in ancient times, I’m just telling you how it was). However, Rebekah would not be left on her own. Part of the birthright is the responsibility of taking care of female family, which includes Rebekah and any unmarried sisters (we don’t know of any sisters to Esau and Jacob, but the tradition stands). This is part of the reason for the extra share of possessions in the birthright, to offset the added responsibility.
Because Esau sells off his birthright for so little, it’s clear he doesn’t value it the way he should, while Jacob clearly does. It may seem opportunistic, but Jacob is behaving responsibly to protect his father’s legacy and to take care of his mother.