They blame themselves for not acting fearlessly anyhow.

Thus conscience does make cowards of us all, And thus the native hue of resolution Is sicklied o’er with the pale cast of thought, And enterprises of great pitch and moment With this regard their currents turn awry, And lose the name of action.

This short example illustrates a broad story: 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.

When people reflect later in life, it is the things they did not do that generate the greatest despair.