Ever wondered that we can remember past but, are unable to do so for future. Why we perceive the time’s arrow moving only in one direction that is from past to future and not the other way round? The answer to this could be explained by entropy of a closed system. But, before we get into into the answer, we should know a little about notions of time. According to Stephen Hawking, there are three notions or arrows of time: psychological (that we perceive through brain), entropic (movement in nature from order to disorder) and cosmological (it moves forward or backward depending upon the inflationary or deflationary state of the universe). Psychological and entropic arrows of time could be considered same and here we are discussing the reason that forbids us from remembering future in the context of psychological and entropic notions of time.
In a closed system, changes take place in a direction that increases entropy. Now, what is entropy? In simple language, we can say that entropy is the degree of unpredictability of a system. In case of Universe, we are on a constant move that is increasing the entropy or disorganisation within it. It is the reason behind the flow of heat from a hot to cold object but, not vice versa. Our memory is created by the particular fashion in which neurons orient themselves within our brain, energy is required to create this orientation which in turn heats up our bodies, thus increasing the entropy. So, the reason behind our perception of uni-directional psychological notion of time is that it creates increase in entropy.
Read more on it: http://www.math.cornell.edu/~numb3rs/luthy3/thearrowoftime.html