Why do people regret actions they didn't take when looking back on their lives?

• Is it true that individuals often regret recent actions (i.e., last few months) but when they look back on their lives, they tend to regret things that they have not done (or lack of actions) ?
• If so, what causes this?
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(Technically, I don't think the title says what you want to express. You probably mean they regret actions they didn't take. If the decision is about doing something or not doing something, then that's a dicision they necessarily have to make. If on the other hand, there is a decision between two things and they didn't choose either, then they don't regret the decision but they regret not making one. That is, the decission is not the subject of regret, but the action not to make it. (v2)) –  Nikolaj K. Oct 16 '12 at 9:08
@NickKidman Yes, I had some trouble expressing it, anyway, I think actions is more accurate, so I will edit the question. Thanks –  mux Oct 16 '12 at 11:41
I guess the referent (now just being actions istead of decision) is still not correct, but anyway, i think people will understand it. ;) –  Nikolaj K. Oct 16 '12 at 11:59