I wrote a long post on this a while ago so I'll try my best to summarize it.
Basically, Sasuke is the driving point of this manga. No, he's not the main character and at the end of the day Naruto is the hero, but undeniably, Sasuke drives the plot. He was Naruto's "goal" ever since the end of part 1. They progressed as a pair because Naruto sought him out as a rival to fight against. As a brother as well. Sasuke's character is very complex. Right from the beginning being manipulated by Itachi, Sasuke's ideals change based on how close to the ultimate truth he is. When he was in the dark about Itachi's motives, he had a hollow and simple goal of killing Itachi to avenge his clan. As he got closer to the truth, his "plans" began to change and would change in complexity in scope. Say anything you want about his emotional antics, that's a realistic representation of how a human would react after they killed their brother who killed their family but later to find out he was just trying to protect you and by extension, the entire village that forced him into doing that. That would psychologically destroy someone. And continuing on, he felt the need for revolution after finally learning about the root problems of the Shinobi world which was first catalyzed by Hashirama and then described by Hagoromo. Whether you like it or not, Sasuke's motives/actions are the reason this manga was so interesting. The complexity, the unpredictability. It made it so interesting and he is one of the best if not the best character in this manga.