Luck has been associated with "happiness" in many cultures across the last few thousand years. There is a concept of management accounting of "rewarding for b while hoping for a". I think the soft focus that allows for creativity helps "luck", but everyone who "knows better than you" wants to proud out good behavior by rewarding for b.
Though a more precise variant that specifically means "lucky" exists (udachlivyi, meaning possessing greater chances), schastlivyi is more often used to describe a person, with the semantic content being that they are both happy and lucky.
For example, the famous Tolstoy quote about unhappy families being different and happy families all looking alike uses schastlivyi.