The human brain is like a computer.
It has the power to remember millions of things. However, we only choose to remember the things that are important to our lives or say something about who we are. For example. when you were a child you rode your bicycle hundreds of times. but you may only remember the first time you rode it. or a time you had an accident. You choose to remember these times, and as you grow older these memories become the story of your life.
The interesting thing is that you can change the details of your memories. We can remember things in a different way from how they happened to make them more important. For example, a child who was chased by a small dog may remember this dog as a big dog many years later. Our memory is not always true. Sometimes it's just how we want to remember the past.