Arashi was about to say goodbye to his sensei before he got cryptic about something that he needed to ask him. What was the question and why would it make him uncomfortable? Then the bomb dropped. When Akio mentioned Sasuke dying to protect him he looked down and grabbed hold of his shirt where his heart was. He'd never forget being told that Sasuke had died against Pain. It really made him feel horrible and he continued to wonder if things would have turned out differently if he hadn't run away.
He almost gave himself whiplash when he lifted his head at Akio mentioning the Mangekyou Sharingan. He didn't unlock it and to his knowledge he didn't even unlock the third tomoe in response to finding out. He knew he wasn't heartless, but there had to be a logical explanation as to why. He mustered one up in his head and looked to Akio. He shook his head and activated his Sharingan to reveal that there were still only two tomoe there. "I think I didn't unlock it because I only found out that he died when he came back to life. If... if I had witnessed it... then maybe something would have happened."
He had to believe that was the case. If Sasuke had died in front of him he would have awakened his Mangekyou and would have been able to get revenge right then and there. "I don't want to be put in a situation like that again, sensei. I won't leave another comrade behind to die. I won't leave those precious to me."