The responsibility to fulfill his parent's expectations that Tony felt before his brother's departure is multiplied when Tony witnesses his father's reaction to the brothers leaving.
Already caught up in the conflicting wishes of his two parents, Tony becomes the last one that is capable of carrying them out.
Additionally, Tony sees that his father was relying on his hopes for a life on the llano with his brothers. Having given up that life for a time, he has been deeply hoping to take it up again with his sons. When they leave, Tony sees how shaken his father becomes. A big part of his being is taken away from him - the part that held closely to hope.
Tony also finds that, as a result, his father is changing and softening his desire to roam the land.
Gabriel, Tony’s father, hopes for his sons to become like the vaqueros, or cowboys, of his family.
Gabriel realizes that the freedom that draws upon him in his blood to move to California and roam the planes is the same freedom that called his sons away. This realization leads to a change in Gabriel that Tony notices. Toward the end of the novel, Gabriel speaks to this change in himself, acknowledging a new desire to compromise his values and visions with those of his wife.
“We have been at odds all of our lives, the wind and the earth. Perhaps it is time we gave up the old differences.”