I've heard "good for you" so many times from native English speakers. I guess it's really common to say. When do you say it?

Native speaker’s answer
Rebecca
Yes, it is common. In this context, you use it when you want to express happy emotions. It can mean "congratulations" or "that's great." Ex: A: Hey, I got the job. B: Oh, good for you! Ex: A: I think I'm finally losing weight and starting to get in shape. B: Good for you.