We have yet to see. Perhaps the reason could be their bond of friendship. He's unable to sever it because Naruto keeps coming back and he hates the fact that his ties to Naruto make him vulnerable. Remember he always wanted to be strong on his own.
"Bonds do nothing but slow you down and distract you from your real goals."
I think this is the closest to his motivation. Sasuke has stated repeatedly throughout the story (and that is actually confirmed by Tobirama's explanation on the origin of the Sharingan power) that pain is the source of his strength. The greater the pain, and the stronger the bond that he cuts is, the more powerful it will make him. With Itachi's passing, Naruto is the strongest bond Sasuke has, and Naruto clearly will not let him cut it without a fight. Cutting off all his bonds will erase the past - in some ways, similar to his stated goals when he last joined with team 7, after he killed Danzo. He had Itachi's eyes put in just so he could be at max power to defeat Naruto. Moreover, Naruto is an obstacle to his new goal (to become Hokage), though I can't imagine that he thinks that he would get chosen for the role if he kills the village hero. This implies that he seeks to get that role by coersion (either direct, or through occular jutsu).
Anyway, to kill the past and make a fresh start he has to kill the only remaining bond to his past - Naruto.