It seems to me also that these are what they are or wanted to be - Ennis thought of himself as a cowboy and Alma asks, when Jack's postcard comes, "is he someone you cowboyed with?" I think "cowboy" is more romantic, Western, macho, than temporary ranchhand, which is what Ennis was.
Same with "Rodeo" - LD uses it as an insult, someone who's an entertainer, but in fact it's what Jack wanted to be, what his father was a success at ( as he wasn't at ranching) and I guess it had a sort of non-conformist show-biz sort of glamor about it to Jack - being a tractor salesman doesn't sound half as interesting.