It could have been taken for Ennis as an insult or,at least,as a renounce of the part of Jack of this "sweet life together".This,joined with the suspicions he had about Jack's faithfulness,made him answer in this way.
But I tend to suppose that,mainly,he was suspicious,yes,but he was waiting for the right moment to tell it.He knew perfectly that he couldn't "attack" Jack with this suspicion after having told him that they didn't see each other in months.But his friend "served it in argent plate",as we say here,when he said this "I did once"; Ennis then had a good excuse to avoid the subject by speaking,at his turn,about what happenned with Mexico.JMHO.
yes, Ennis certainly must have taken this for an opportunity to unload his resentment regarding his suspicions about Jack's faithfulness.
As to the sweet life, I wonder whether Ennis would have cared that much. I think what he resented most was not that Jack was renouncing it -- rather, he resented the fact that he was somehow being blamed for the renouncement.