I'm soooo in the Jack would never quit Ennis camp...I agree completely with stacp that "Damn you, Ennis" meant he could never walk away from this hurt, sweet soul.
And, yes, we've all let off steam saying we'll never talk to this person again, or have dinner with that person, etc...but it's just that usually. Jack tells his dad about Randall but he never follows through with it...and when he's complaining about August and always being in the cold, he says " we oughta go to Mexico."...so Randall wasn't in the plan at that moment.
When Ennis drives away that last time, Jack knows that this is his life...always waiting for those precious moments with Ennis.