After being rejected so often by Ennis, Jack may have seen Randall as at least another possibility, and a more acceptable one than staying in an empty marriage with a woman more obsessed with money than her family. Randall made the first move, showed an interest ( as Jack once did with Ennis), was available, relatively near, and camouflaged by being married to a sort of Southern belle type- LaShawn reminds me of some of the dimmer females in"Gone with the Wind" - didn't have any of Ennis's complications. when ennis wasn't there and wasn't going to be there for lengthy periods, maybe Randall just filled the gap and didn't seem too bad until Ennis was there again, at which point he faded into insignificance. They led pretty compartmentalized lives, after all.