Canada's Globe and Mail has the answer:
“On average, bronze medalists are happier than silver medalists,” said Victoria Medvec, a psychologist and professor at Northwestern University's Kellogg School of Management in Illinois.
The phenomenon is a case of counterfactual thinking – thoughts about “what might have been,” she explained.
Third-place winners have upward thoughts (“at least I won”) that increase satisfaction, researchers have found, whereas those who come in second tend to have downward “if only” thoughts that decrease happiness.
The most telling study involving athletes used footage from medal ceremonies at the 1992 Summer Olympics in Barcelona.Researchers including Dr. Medvec asked subjects to rate the satisfaction of bronze and silver medal winners based on their facial expressions. The study revealed a disconnect between performance and satisfaction, said Dr. Medvec. “Those who perform objectively better can actually feel worse than those who they outperformed.”
Expectations from sponsors, teammates and fans can contribute to an athlete's sense of disappointment, according to Saul Miller, a Vancouver-based clinical psychologist and author of Performing Under Pressure: Gaining the Mental Edge in Business and Sport.