The two have a falling out over the course of Shippuden, so why did Sasuke smile when Naruto beat Pain?
Sasuke has to smile when he hears that Naruto beat Pain because he’s proud. The two have had a rivalry their entire lives, and the idea that Naruto is actually surpassing Sasuke in strength is incredible. Sasuke smiles out of pleasant shock and the possible thought of their inevitable battle becoming even more difficult.
In the past, Sasuke realized that Naruto is stronger. Yet, as teenagers, it still impresses him that Naruto is able to keep up.
Sasuke decides to smile after hearing that Naruto beat Pain
In chapter #453, Sasuke and his group are approached by Obito, an Akatsuki representative.
Sasuke informs Obito that Team Taka, his group, has left the Akatsuki. The business has been concluded following the team’s capture of the Eight Tails jinchuriki.
Obito informs the group that the Eight Tails used a substitution jutsu, and he is still at large. Sasuke thinks back to the moment when he slices off the Eight Tails’ tentacle, assuming it to be the instant when the substitution occurs.
Since Team Taka’s job is technically not completed, Obito arranges another mission for the group to fulfill their obligation to the Akatsuki. Following a failed attack by Sasuke, Obito calmly explains the demise of the Leaf Village.
The Akatsuki member Pain, Obito explains, destroyed the village before suffering defeat by the hands of Naruto. Obito also emphasizes the importance of an upcoming Five Kage summit. The meeting will provide ample opportunity for Sasuke to exact his revenge on Danzo, the man who betrayed Itachi and forced him to destroy the Uchiha family.
Obito says that Naruto became “extremely strong” in order to defeat Pain. “In fact,” Obito says, “I think he’s stronger than Sasuke right now.”
In the manga, there actually is not a panel of Sasuke smiling during this moment. The smiling was added to the anime only.
But why does Sasuke smile? Well, he’s had a rivalry with Naruto his entire life. Hearing that Naruto has become so strong that he may have actually surpassed Sasuke is incredible. Sasuke smiles because he’s proud of Naruto’s growth and he anticipates their inevitable battle will be even more epic in scale.