It's good to appreciate others.
Dave is told by Marsha that Dave's boss Tony, who Dave hasn't liked in the past, went out of his way to make Dave look good to upper management.
Dave wants to respond to the news he just heard.
Dave tells Marsha that he's amazed by Tony's generosity and hopes that he can find a way to thank Tony and repay him.
Dave tells Marsha that it's about time that Tony did something useful as Tony is generally useless.
Marsha gains respect for Dave and is happy to see someone willing to give praise to a person they didn't get along in the past.
Marsha is taken aback at the fact that Dave is seeking to point out negatives and doesn't seem to care about Tony's compliments.