When I was at Detroit Country Day we almost lost a game in state playoffs due to a kid having that reaction after missing a goal scoring opportunity. He was in on a breakaway and the keeper made the save, and countered to the side of the field our player came from. Luckily the other team also missed their goal scoring opportunity, but they wouldn't have had a chance if our player recovered quicker rather than try and act like a pro.
While we are on the topic of reactions, there is another aspect of this that troubles me with youth players. In a sequence of receiving, dribbling, passing, and finishing, the players can do everything to near perfection, but if they miss the shot their reaction is as if they just missed a PK in the World Cup final. Conversely, they can have a terrible first touch, sloppy control, an inaccurate pass, but if they hit a perfect ring target in training, they feel they need to brush their shoulders off. I try and nip that in the bud every session with players.