"I promise I'll work so hard" - "Damn it, this is a mistake already!"

Hi.

In a scene where Hilary Swank's character and Clint Eastwood's one debate about hiring Swank for training, she says to him in ecstasy "I promise I'll work so hard" and he goes "Damn it, its a mistake already!" but then I wonder...

What was she supposed to have said instead? And don't most coaches want and demand for their students to be very devoted and hard-working and slack them for even appearing to be lazy? So why does he state "this is a mistake already" like that instead of either "you're welcome" or "you sure? OK let's see", why was hearing that phrase in that moment for him a disappointment?

Thanks.