What does the sure mean

Keith93: Yo, I wanna die before I'm old

Nik2013: And why the fuck would you want to do that?!

Keith93: Becuase I don't like the idea of being wrinkly and fragile, or having a hunched back !

Nik2013: Because*. And stay in shape so you don't look like that in the future.

Keith93: Nik, you know damn well I hate working out.

Nik2013: Okay but for real, that's the only reason why you wanna die before you're old, right?

Keith93: Yuh sure

Nik2013: What does the sure mean

Nik2013: Keith