We should look to the past only to learn from the mistakes we have made and the lessons we have learned. We then apply this knowledge to the present (present has also been defined as a gift).
We may also look briefly to the past in consideration of fond memories and pleasant occurrences. These brace us to face some of the unpleasantries we often face. This should only be used as a tool to help us get past current difficulties, and not as an anchor which keeps us eternally in the past.
We may consider life as a constantly flowing river. The waters of that river never pass the same spot twice. They are always passing something new, and never return to that which they have already passed.
Living in the past is comparable to living in a sealed pond, where there is no influx of fresh waters. The pond becomes stagnant and covered with scum. Our lives are the same when we insist on living in the past.
We need a constant inflow of new ideas and circumstances to stay fresh and continue our growth. Even thought some of these new ideas and circumstances my prove painful, they are essential in our development. When we cease to grow and change, we cease to live life to the fullest.
And...Who wants to be stagnant?