Perhaps it's nature's way of letting us put things behind us.

Spang on, Jo, I'll bet. Good thinking! And yet--mayn't it not work the same for everyone? Can't we choose, at least to some extent, what to recall the clearest? For me, when I have good life-experiences, and think about them (which I do almost constantly), it just makes me look all the more eagerly to see what neat new thing is coming next. It doesn't stop my motivation to achieve.

Is there an "opposite" for the term 'vicious circle'?
Exuberant circle, maybe? Whatever, I think it works in the same pattern as a vicious circle does: each happy instance reinforces the feeling of happiness, which in turn attracts more happy occurrences.