think Jack's wonderful personality came from Mrs. Twist. I think his father was a cold, insatiable prick of a father who had no substantial love for Jack. (I mean, there's a possibility he did but who knows...) Instead, I interpret that Jack overcame his distant and harsh father to become a caring, spirited, sweet, and, like you said, valiant person. (With help from his sweetheart mother.)
But how is waiting for Ennis to come around stubborn?
In real life we always develop personality traits that come from both of our parents, and the education they give us. We learn from the two of them, for better or for worse, if both are always around us, of course. That's why I think Mr. Twist might have also played an important role in Jack's personality.
As for waiting for Ennis, I think 20 years is too much time to wait for anyone to come around. I mean no matter how great your love for that other person is, it's always important that you love yourself more. That's one of the lessons I learned from the movie. And Jack learned it too, remember what his father tells Ennis about his plans to move to Ligtning Flat with another fellow. But maybe he learned it too late, though there was no way he could tell he was going to die.
Jack loved Ennis more than he loved himself. And while that can sound romantic, because in Hollywood they have made it sound romantic, in my opinion it was a mistake. In real life it's a situation that causes a lot of pain and can lead anyone into a depression. Jack at the beginning of the movie was an outgoing happy man, with a quick laugh and a good sense of humor. In the end he loses all that and becomes a bitter, depressed man. And that's not fair, nobody should do that to themselves. That's why I think his stubborness, in this case, was a flaw. He should've loved himself more no matter how much he loved Ennis.