What happens only one time is less impressive to our consciousness than the facts we permanently experience in our life.
We may rightly say that a couple of events define our entire existence, but they do not enter into our permanent consciousness which is always filled by the maters concerning the ways of dealing efficiently with the trivial duties of daily life.
From this reason, we rather credit the unique events with the power to slightly direct our consciousness to follow some goals which are related to them. For example, a unique event which meant the beginning of a great love may determine our consciousness to be preoccupied with different duties expressing our care for the beloved person. But to take care for someone is in fact a set of repetitive facts which is far away from the original unique event of falling in love.
Moreover, there are some unique events which hardly can be linked to any similar goals of the consciousness, especially, the tragic events of losing friends or relatives. The consciousness does not find any simile in the course of daily life. Therefore, such events are easily forgotten or become strange echoes when the man has not daily duties or his consciousness is less active during his dreams. They are strange echoes, because they claim to change the consciousness as any other unique event and they are refused by us, even if they had been precious moments of our life.