"Flowing water never goes stale" is an old Chinese proverb. This statement reflects many meanings. The more we learn, the more we can advance. We as humans change day to day, whether it be in mood or even on a cellular level.
The Buddha taught that we are not the person we were yesterday. We learn, we grow, our body changes, our mind changes, etc. Holding onto the past is attachment to something that physically does not exist. Ideas and attitudes are mere phenomena which hold no form. Attachment to ideas, attitudes, and expectations only creates more suffering. When we don't get what want, what we think is right, what we think should happen, we get upset. When something negative has happened in our past and we keep "going back" to it, that is also attachment.
The past is meant for reflection, not to be "lived-in." It's already gone, forever. We can definitely learn from the past, use those lessons in the present, and see what may manifest itself in the future. A pond has many forms of algae and bacteria in it because it is stagnant. Our minds are very similar. When we ponder and attach to things, we grow ideas and attachment, which to the mind are just as bad as bacteria and algae. Keeping the mind fresh and moving within the moment is a way to avoid going stale and accumulating these "growths."
Even as a flowing stream encounters a rock or obstacle, it doesn't cease its movement. Our minds and ourselves should act as such.