I don't own Naruto.
Feel
Kakashi thought you know how strong a person is by how long they'd stayed alive.
Sakura thought you needed knowledge to be a good ninja.
Naruto thought you were unbeatable if you had a lot of chakra.
Sasuke, however, knew that you needed to not feel. Not just 'not show emotion', as Sakura thought, but to not feel at all, and pretend to show feelings that you had never experienced.
You needed to be a shell of life, just existing to complete your goals. How could you kill with a heart full of love? How could you not lose sight of your goals with blinding fury?
This was how Sasuke could sneer at Itachi. Not enough hate? If Itachi felt hate, he was just another person. A normal person who could easily be defeated. Naruto was too full of happiness and hope. Hope can be crushed, just like the frail blond boy. Sakura had too much innocence. Although this wasn't an emotion, no way could a child with loving parents and a caring nature have any strength at all.
Kakashi was his sensei. Kakashi could be ruthless.
That time he was struggling under the man, Sasuke realised there might be a lot to learn off Kakashi. He felt no emotion coming from the man through the whole time Sasuke was underneath him.
But in time, the Uchiha realised this was no more than a facade. The man was faking not having emotions like Sasuke was faking having them.
When he found this out, Sasuke was ready to go to Orochimaru, ready to drop all pretence of emotions.
If only he could feel.