When you feel disappointed, you are feeling deprived of something you thought was already in motion. If you're feeling like you have an "appointment" with a promotion or a successful presentation or a new love, having that thing not work out is especially crushing because it was kind of a done deal inside your mind.
The mother at the zoo was lucky; my situation could easily have been hers. Children are fast and clever little beings, who can get out of your grasp in a second. Trust me, no matter what harsh things you think you are dishing out, the mother would be delivering even harsher ones to herself on the inside. She doesn't need to hear anything more from strangers, she knows.