Both Ennis and Jack have an opposite attitude in theirs lives and in the way they handle their love story:
Ennis, poor has nothing to lose, free and lonesome after nine years with Alma. No social position to jeopardize, but he remains locked in his refusal attitude to share Jack's life
Jack has a good job, good life, is weathy. Not for free, for this improvment he had to bear the contempt of the familly. In another hand he is ready to throw this all away easily (after Ennis divorce he drives straight, believing on his chance to make his dream come true).
yin and yang again.
I focus on that, thinking this opposition makes them complex, deep, so close to us (to me indeed!)