I don't think Jack expected anything about Randall, when Randall talk him of the little cabin he does not seem happy or excited, he is sad, melancolic! Since the misunderstanding after the divorce Jack was wounded and never could recover, we never see him happy after, never a smile. Ennis and Jack were in love, deeply in love, but more than that they were life for each other, and Ennis could not bear that. Before their meeting, on Brokeback, they were both nearly dead, they came back to life in fiding that love... After that they were not able to live with or without the other...
Maybe Ennis was realy in love with Cassie, she made him dance! After the last fight, along the lake, Ennis is broken, he is lost for everybody but melancoly. And Jack! what I think it is that the thing he cant bear it is not being able to help his friend, to make him happy. It is not only a question to declare love, they are both unable, they are crippled! Jack was happy as long he thought he made Ennis happy, just look his smile when they they meet at mud lake, just before the ballad in the rain forest! Jack is a worn out man, his last try with Ennis is hopeless. I think he chose to die, he tought he could do in dying what he could not in living. That is how I understand the memory of the happy time in Brokeback, Jack is sure of his love and he is sure of Ennis: he prefer let him go, be free, "see you in the morning!..." Jack was not murdered, he was not stupid, he gave his life and that is what Ennis understand in the closet. When Ennis wisper "I love you" it is not only him, he just came to understand what Jack did for him. Randall is a nothing, Jack talked of him only to get the suspicion out of Ennis... I think the trailer is completly out of the way, love is NOT of force of nature, love between Jack and Ennis was built day after day, love is a bond and there is no greater love than give your life fore someone you love :\'( :\'(