Because it reflects into the future?. Everything bounces around sometimes.
If past was bad you could think things won't get any better... maybe discounting any options and so on, ignore the more positives and possibilities.
If past was good you could be afraid of it slipping away.. or that things will just go bad.. etc.
Reminds me of this..
"We never keep to the present. We recall the past; we anticipate the future as if we found it too slow in coming and were trying to hurry it up, or we recall the past as if to stay its too rapid not to belong to us, and do not think of the only one that does; so vain that we dream of times that are not and blindly flee the only one that is.
The fact is that the present usually hurts. We thrust it out of sight because it distresses us, and if we find it enjoyable, we are sorry to see it slip away.
We try to give it the support of the future, and think how we are going to arrange things over which we have no control for a time we can never be sure of reaching.
Let each of us examine his thoughts; he will find them wholly concerned with the past or the future. We almost never think of the present, and if we do think of it, it is only to see what light it throws on our plans for the future.
The present is never our end. The past and the present are our means , the future alone our end.
Thus we never acually live, but hope to live, and since we are always planning how to be happy, it is enevitable that we should never be so."