I think Ennis's view of the world involved how he was expected to be useful to others. His socialization was extreme and it was made clear to him that his needs and desires were of no importance. He saw himself as a minor part of a larger picture and he had obligations to his family, Aguirre, Alma, his kids, the courts, his ranch bosses, Cassie, and all the people on the sidewalk in town. His wishes and desires were unimportant.
His life was based on survival and not on comfort, happiness or fulfillment. His meals, like his other consumptions, were spartan - but he spent little time bitching or expecting more.
Love was just another four letter word to him.
In the novella Annie Proulx uses the word 'love' for the love Jack had for a little dog, the love that Ennis had for his daughters, and as a closing to Alma's suspicious note to Ennis.
The closest to love expressed to Ennis in his early life - that we see - was his mom telling her little cowboy that it's time to hit the hay. Time to hit the hay and be ready to 'work' again tomorrow.
So when Jack Twist came along and connected with Ennis, it sparked him into being a more complete human, but no more aware of love as a thing that he deserved on a daily basis. Yes Ennis enjoyed the social and sexual activity with Jack, but it was like whiskey, round steak or elk meat. Enjoy it while you have it, but don't expect to have it all the time. There was comfort in the closeness to Jack, but his personal comfort did not pay the bills, satisfy the boss or the wife or please the people on the sidewalk. Being with Jack was a guilty pleasure for Ennis.
On that fateful day when Ennis got the news:
Ennis didn't know about the accident for months until his postcard to Jack saying that November still looked like the first chance came back stamped DECEASED.
Suddenly he knew that his guilty pleasure wasn't there anymore. And it never would be again. Even after that, does Ennis feel pangs of love for Jack, or just the hunger for a bit of steak once in a while? He knows there's a hole in his life. He knows Jack valued even his old shirt. He knows that you should marry the person you love, not the person appointed for you by circumstances or families.
Even then does he know that the feeling he has for a MAN called Jack is a kind of love? Can he experience love as a fine unfiltered thing between two sharing humans? I think more than the vocabulary is missing. I think Ennis has the phantom pains you have when an arm is missing, but you don't have to 'love' your arm to miss it and to know that you need it to do the day-to-day things that make life worthwhile.
Ennis loved Jack from Day One.
But what is love if you deny it all your life?
But what is love if you feel that you don't deserve it?