Answer: Broaden and build theory
Explanation: Broaden and build theory describes about the happiness and joy is responsible for extending one's interaction and awareness to think, improving creativeness, enhance actions etc. It determines and encourages about the positive and happy emotion.
The observation mentioned in the question is indicating about the broaden and build theory of positive emotions. It is describing the happiness as the source of open thinking, looking positively on those events that were overlooked before etc.