She's looking back as though to say (in a nice way) "What are you looking at? He's mine and I'll do with him as I please." The boy is very fixated and hot & bothered about those two big breasts in front of him. But at the same time, he knows that he can't just reach over and grab what's in front of him. I don't even think he was expecting to have this problem right now. I could also see him experiencing inner conflict.
It's a nice picture to look at.