Hi,
This is my 2 cent worth, Ennis and Jack on the surface was a ranch hand and a cowboy, outdoors men. drinking was part of the life. I don't know if they were functioning alcoholics, but it was part of them. When Ennis was thrown from the horse, and came back to camp he was mad. Jack was drinking because he was upset there was no dinner, only those darn beans. Ennis said " Isn't there whiskey or somethin", He didn't want any sissy drink like water, heck he wasn't fainting.
On the night of their first encounter they finally opened up, like maybe two good friends in a bar, and talking about what they believed, feeling each other out on some things. When you go out with a friend, a lover your guard is down you're relaxed, comfortable so you open up, act silly even.
Alcohol was just a part of their lives.