Remember how Kaz always makes jokes about how he's dumb, and reckless, and can't do things? And how he says all of this as a joke?
Well, it's not.
He's not.
Joking, I mean.
Kaz has really low self-esteem. It doesn't help that whenever he tries to do things, Oliver always gets the attention
It's not his fault; he only does those things so someone would pay attention to him for once.
At home, he has so many siblings that his own parents don't know his name. It doesn't help that they get mad sometimes and lash out; they do t means to, he knows that, it's just that they have to vent out their frustrations somehow and he can't let his younger siblings get hurt, because he's the oldest except for the brother that's in jail, and his parents don't even care!
So, yes, it hurts when he says those things, and when other people say those things. He tries to please others, he really does, but when that doesn't work, getting on their bad side is the only other way for them to notice him.
Like when he threw that surprise party for Oliver. Each time he called Kaz selfish stung, like a wasp, right in his heart.
It hurts, when he gets a failing grade because his teachers don't listen to him
When Oliver, Jordan, Skylar, Gus, or one of his other friends puts him down.
So yes, so sorry for being stupid, dumb, selfish, annoying, reckless, stubborn, or whatever.
Because Kaz knew in his heart that he was a hero, and nothing was going to change that.
(Really, he didn't. He was a failure, through and through, and his parents made sure he knew that, if when they pulled out the whip was any sort of indicator. But if he didn't joke, he'd cry, and break down. And he can never do that. He had a reputation to uphold, after all.)