For example, we let ourselves forget the unimportant, painful, and embarrassing things like old phone numbers or youthful indiscretions. (We hope these things are lost in time and space, or at least piling up in some distant, high security junkyard.)
We make ourselves remember the practical, the pleasant, and the rewarding, such as our salary or the high points of our childhood. While most of us have no problem reciting our own social security number, we often forget anniversaries and birthdays. We reinforce our strongest memories by remembering and re-telling them, until they become almost alive.
Planning--the psychological act of deciding on goals and organizing future steps to achieve them--as practiced is a process of filtering out the stuff to forget, and arranging the good stuff that remains.
It is no wonder that plans are so often full of promise and potential. We think, "This time, we will do more, in less time and with less money." The forces that impeded us then are vanquished now. We have only to try harder and work smarter to meet or surpass our goals.
Modern planners employ 'fudge factors' or 'pad' the estimates to reflect the reality of not plan-able delays and barriers; but they are only fooling themselves. Planning boards and approval managers will never accept truly realistic plans; they will only approve overtly optimistic ones. We have institutionalized our tendency to forget what we know, and ignore what we have learned. (A strong change control process helps make up for this.)
Like Sisyphus, the mythical Greek king trapped by a boulder on a mountainside, we are stuck repeating the same biased decisions, over and over again.