regret
that things don't always go the way you want them to is something that sasuke's starting to catch onto.
he always thinks he could have worked harder, always thinks he could have done something better, but whenever he thinks that, it's always after the fact, after everything is done and over with, and there's no way to go back and fix it.
this is where sasuke goes wrong, every single time. he doesn't work hard enough before things catch up to him, he works hard when there's no time left and no matter how hard he works, it won't count for much, because he always runs out of time. he can't get what he wants, and he's left with the sour taste of mediocrity, because it's not as though he tried his best; he tried his best in the time that he had left, because he left himself too little time in which to try.
sasuke hates the bitter taste of regret, of lessons learned the hard way, and it's not even like those lessons stay. they don't. they sting for a while, and then he forgets.