Jack's father had been a bull-rider too, so he'd have been in a drinking environment.And as I don't like him and I don't like people who drink ( I mean seriously), I've decided he was an ugly drunk - sheer prejudice, this. On the rodeo circuit Jack would drink with the others - as you say, Edgar, part of the culture - and because he's a sociable character.
I've decided that I like Jack because he is a sinner - he drinks and smokes, breaks the rules, swears, lies and is generally the sort of person it would be good to be. Ennis is not a natural sinner - OK he smokes and drinks but often because Jack offers him booze, but he doesn't swear, wants to do what Aguirre wants, is fixated on getting back to the sheep most of the time, keeps his promises and is a general nause. I would not want ot be stuck on a desert isalnd with anyone who at nineteen hasn't already been a sinner.
So there.