Humans tend to be most adversely affected when they are "close". In one study, olympic bronze medalists were found to be happier than silver medalists. Those in third were happy to earn a medal, while those in second regret that they could not have pushed themselves further to get gold. Another study compared too options for a raffle rewarding positive behavior. In one, those who did what they were supposed to had a ticket in the raffle. In another, everyone had a ticket in the raffle, but those that failed to complete the task did not get the prize. The second approach was much more motivating. People were more upset that they could have received the prize, but failed to do what was needed.
Regret of commission is also different than regret of commission.
On tests, people regret switching from incorrect answer to correct answer. However, the regret is not as strong for failure to switch from incorrect to correct answer. (Despite conventional wisdom, it was found that people that switch answers tend to do better.)
Some people try to maximize experiences. There is a quest to avoid regret. (An example is Jeff Bezos and starting Amazon to avoid regret of not doing it.) This can be a negative in which we spend too much time avoiding possible regret that we have difficulty making decisions. (Booking travel always seems to fall into that realm.)
Regret assumes agency. If we cannot control something we cannot regret the outcome. If something is 100% out of our control, we do not regret it. However, there is almost some point where we do have control. (We may have chosen not to travel somewhere, thus avoiding an earthquake. ) By optimizing regret, we can improve our lives.
When posits the thesis that "when" you do something can be as a more important than how you do it (or what you do.) Sometimes, bad timing can result in significantly different outcomes. Somebody that begins their career during a recession is likely to end up significantly behind somebody that started during an economic boom. Similarly, time of day can impact the decision making process. (Perhaps the Lucitania was sunk because the captain was not thinking well during the afternoon.) Different people have different times of peak decision making ability. Finding the time can help to make better decisions. Naps can also be very helpful in increasing productivity. (The author laments the ending of the Spanish siesta.) People like to hear the "bad news" first before hearing the good - yet we tend to preface bad news by telling the good. There are many other areas that can be helped by paying head to "when" things are done.