Why is Atticus so affectionate toward Jem after Jem has disobeyed him in To Kill a Mockingbird?

In Harper Lee's novel, To Kill a
Mockingbird, Chapter Fifteen finds Atticus sitting in front of the jail where
Tom Robinson is being held, keeping watch. The reason is soon apparent as a mob of angry
men arrive, demanding Tom's release into their hands. It is obvious that these men are
ready to lynch Tom. As Atticus prepares to face the angry group, Jem, Scout and Dill run
up and join him.

It is easy to understand why Atticus is so
frightened that the children are there: he fears that they may be harmed, and he tells
Jem to take the other two kids and go home.

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…by the way he stood Jem was not thinking of
budging.

"Go home, I
said."

Jem shook his head. As Atticus's fists went to his
hips, so did Jem's, and as they faced each other I could see little resemblance between
them…Mutual defiance made them
alike.

It doesn't matter if
Atticus demands or pleads, Jem will not leave his father's side. It seems to me that
more than anything, Jem wants to protect his father, and he takes a stand. Atticus is
afraid, but Jem is too young to know to be afraid. However, Jem is
of an age to grasp the importance of supporting his father, as his father tries to do
the right thing in protecting Tom. I don't even know that Jem understands the
implications of the mob's demand, but he can certainly comprehend
the angry demeanor of the men who are confronting his
father.

When Scout has had a chance to speak to Mr.
Cunningham, helping him to remember that like Atticus, he is a father, Mr. Cunningham
disperses the crowd. As they start to walk home, Atticus and Jem walk ahead, and Scout
is sure her father is yelling at Jem for his disobedience. She realizes she is wrong
when he ruffles Jem's hair… 
"his one gesture of
affection."

I believe that Atticus, while frightened, is
more than a little proud of his son for standing by to support him. Perhaps Atticus is
realizing that Jem is becoming a young man, putting his childhood behind him. It may
simply be that Atticus realizes as they go back to the house, that they are two men
walking—united in purpose—rather than a man and a boy.