Here's my opinion of what are Naruto's and Sasuke's motives and there whole character development in the series as a whole.
Naruto
In my opinion Naruto's entire motives are this. He wants to challenge the impossible. Look at all the different aspects, all the different events. All Naruto tries to do is the impossible. So Sasuke is not a different case.
Naruto wants to bring Sasuke back, because it's one of those things that everyone has told him was impossible. It's like his fight with Neji. Neji told him it was impossible to win, so he challenged it to win. But Sasuke is one that he hasn't won yet. So that's why he has a psychological obsession with getting Sasuke back. So he can win the impossible task.
Sasuke
Sasuke doesn't want to avenge anything, he doesn't want to avenge and he's not a avenger. He just want's to not feel powerless. All you have to do is just look at the past in the series. When he was a little kid he saw his clan killed and he saw Itachi do it, but he couldn't do shit because he was powerless. Because of those events, he has this complex where he doesn't want to ever feel powerless. He doesn't want anyone to hold that much power over them like the carnage Itachi pulled off and he couldn't do anything. Sasuke want's to have all the power and be untouchable, so nobody can ever do what Itachi did to him.
Now, this could all be wrong because this is in fact a shounen series, and Kishimoto might not be planning for this series to be this psychological.. but if he is then that's good writing, but again it's Naruto so who knows..
Naruto
In my opinion Naruto's entire motives are this. He wants to challenge the impossible. Look at all the different aspects, all the different events. All Naruto tries to do is the impossible. So Sasuke is not a different case.
Naruto wants to bring Sasuke back, because it's one of those things that everyone has told him was impossible. It's like his fight with Neji. Neji told him it was impossible to win, so he challenged it to win. But Sasuke is one that he hasn't won yet. So that's why he has a psychological obsession with getting Sasuke back. So he can win the impossible task.
Sasuke
Sasuke doesn't want to avenge anything, he doesn't want to avenge and he's not a avenger. He just want's to not feel powerless. All you have to do is just look at the past in the series. When he was a little kid he saw his clan killed and he saw Itachi do it, but he couldn't do shit because he was powerless. Because of those events, he has this complex where he doesn't want to ever feel powerless. He doesn't want anyone to hold that much power over them like the carnage Itachi pulled off and he couldn't do anything. Sasuke want's to have all the power and be untouchable, so nobody can ever do what Itachi did to him.
Now, this could all be wrong because this is in fact a shounen series, and Kishimoto might not be planning for this series to be this psychological.. but if he is then that's good writing, but again it's Naruto so who knows..