Our brain is considered a computer. It records data and data is kept by our brain without experiencing any changes. Actually, the brain stores memories that we think never change. When we remember something, our brain revives them. However, we may not always remember events like the first time. That's why, our memories are unreliable. There are some reasons why memories are not reliable.
First of all, one of the main reasons for an unreliable memory is that when we want to recall our memories, we reconstruct the events. We are facing a lot of events every day. The new situations which we encounter cause us to reshape the events. In addition, our memories are affected by our present expectations and our current knowledge and thoughts. Thus, while we are remembering the past events, we can mix the present and the past. Therefore, our memories can deceive us and they may be unreliable.
Secondly, another reason for unreliable memories is that we usually suppress memories that are painful and damaging to our self-esteem. We have a tendency to remember pleasant memories because painful memories torment us when we revisit them. Therefore, we may not say that memories are completely true and reliable. Moreover, we are open to external factors such as false memories. We are open to believing in unexperienced events. For example, when a person says “a wild dog was chasing you in your childhood”, you believe the person who tells that story. That's to say, you can be misled. That's why, our memories are unreliable.
All in all, our brain is not a machine which preserves the events and memories. Our brain keeps the memories in mind and each time we recall something, these memories come out changed. Our brain reconstructs them. Therefore, our memories are unreliable.