Their natural reactions of shock and surprise are just priceless. It elicits laughter and happiness from everyone (the one who was pranked included, if they have a good sense of humor). For example, this guy in the video puts a little frog down his girlfriend’s shirt. It looks like she was playing drums in the game Rock Band and it took a moment before the action registered a hilarious reaction.
While all of us are laughing, you could see that the guy was chased out of a frame and probably got a good whooping from his girlfriend.