You regret most the things you did not do.
Thomas Gilovich of Cornell University found that people are twice as likely to regret a failure to act. Why? We rationalize our failures, but we can't rationalize away the stuff we never tried at all. As we get older we also tend to remember the good things and forget the bad. So simply doing more means greater happiness when we're older (and cooler stories for the grandkids). Lucky people don't dwell on failure; they see the good side of the bad and often learn from it.
—Eric Barker, (resource) Barking Up the Wrong Tree