For young children, getting overwhelmed with their emotions is a common thing. It could happen not just when they're angry or sad but even when they're happy - ever see overexcited, maniacal kids running around and screaming with laughter at inappropriate places while their parents try feebly to monitor them? Children, no matter how young, need to be taught the value of restraint. With self-control, they’ll learn to deal with negative emotions and mood swings without sinking into depression or behaving inappropriately in social situations. Sure, there's nothing wrong with kids being excited kids but sooner or later they'd need to know that there's a time and place for everything.
Example: Your daughter had been painstakingly building a sandcastle on the beach when her cousin accidentally destroyed it during a bout of rough play. While she has every right to be angry and upset, the emotion has to be managed well or else she would eventually become the kind of person who would lash out at people or act violently in the name of "eye for an eye". But do allow her to express herself appropriately to her cousin.