In the final scene in the movie, Alma, Jr. visit Ennis to tell him of her upcoming marriage. She was 19, the same age when Jack and Ennis first met.
Ennis begins to question her, and He asks "does he love you", and she only been with guy for a year, and she said "Yes, daddy he loves me.", Ennis then turns to look out the window, what was going on in his mind.
That's the first the word love comes out of Ennis's mouth, the implication of love, the meaning of love, I think he was finding the answer about love more for himself then anything else. His daughter after only a year, knew the man she was marrying loved her.
Did he finally understand then, that Jack loved him, and he loved Jack in return, even if the word love was never said, all those twenty years.
Alma, Jr., made him face something that he couldn't and wouldn't deal himself, he was in love.
If this topic was discussed before, please let me know I would like to read them.