Daniel Kahneman, in Thinking, Fast and Slow. Emphasis mine:
People expect to have stronger emotional reactions (including regret) to an outcome that is produced by action than to the same outcome when it is produced by inaction. This has been verified in the context of gambling: people expect to be happier to gamble and win than if they refrain from gambling and get the same amount. The asymmetry is at least as strong for losses, and it applies to blame as well as to regret. The key is not the difference between commission and omission but the distinction between default options and actions that deviate from the default. When you deviate from the default, you can easily imagine the norm - and if the default is associated with bad consequences, the discrepancy between the two can be the source of painful emotions. The default option when you own a stock is not to sell it, but the default option when you meet your colleague in the morning is to greet him. Selling a stock and failing to greet your coworker are both departures from the default option and natural candidates for regret or blame.
In a compelling demonstration of the power of default options, participants played a computer simulation of blackjack. Some players were asked "Do you wish to hit?" while others were asked "Do you wish to stand?" Regardless of the question, saying yes was associated with much more regret than saying no if the outcome was bad! The question evidently suggests a default response, which is, "I don't have a strong wish to do it." It is the departure from the default that produces regret.