Atticus is proud that Jem notices that no one from Maycomb ever sits on a jury. They are all from the countryside. There are also no women.
You never see anybody from Maycomb on a jury- they all come from out in the woods. (ch 23)
Jem’s observation pleases Atticus. He knows his son wants to be a lawyer and is interested in law. He is proud that Jem has noticed how juries are made up.
Atticus leaned back in his rocking-chair. For some reason he looked pleased with Jem. "I was wondering when that'd occur to you," he said. (ch 23)
Atticus explains that women cannot serve on juries, and people in town would all have a conflict of interest because they might be afraid to sit in judgment of someone else who is a neighbor or customer.
Basically, Atticus is proud because Jem is being more mature. Scout does not understand why Atticus is pleased. She does not realize that Atticus sees Jem as a chip off the old block. He is glad that Jem is smart, but also that he is following in his father’s footsteps and developing a legal mind.
Jem is very concerned when Tom Robinson is convicted. It offends his sense of fairness. This observation shows that Jem is becoming more mature and moving on.