Throughout our lives, it's a natural process for us to store memories about a myriad of moments we live in (which basically divide into "good" and "bad" moments... but obviously they are more complex than this mere separation). They are pieces of our existence that make us who we are... A version, which for good or ill, is always modified (updates in an endless looping).
Thinking like this may sound like a science fiction story, but it is undeniable not to imagine our lives as a movie without pauses and without returns or advances... Therefore, following that philosophy of the "here and now" (which is basically when you live the moment without thinking about what has already happened or what can still happen) where we are the main writers responsible for filling in the endless blank sheets.
Within this scenario, life is playing with us and presenting to all of us the levels that we must overcome to reach ever further (include here a mixture of happiness and sorrow to which all of us are subjected). In an eternal search for happiness, we are building and collecting these memories that shape us as human beings.
So... Remember when you had a surprise party? When did you move in? When did you enter (or graduate) in college? When did you start living alone? When did you first get disappointed with people? When did you get your first job? When did you do so much planning and failed (but also... achieving success)? When did you risk everything for something that made no sense to anyone but yourself (turning yourself into a particular visionary altruist)?
Anyway, do you remember these and many other moments that happened to you and that are part of a vial that is exclusively yours? As much as other people are around us (and sometimes this is very important... after all, no one is alone), helping us build many of these moments, there is a peculiarity in them that makes them something ours alone (yes, no better sense than a sentiment known as selfishness can have)!
What is most interesting in these memories is that once they are stored we can hardly forget them (we can revisit their immensity whenever we want and where we want). There are those who say that this is a good thing ... Some say that this is a bad thing (it is all a very particular vision), but there is a consensus about a supreme idea that humans are only human because they are made of little pieces so rich - and often underestimated by the frivolity of a hallucinated rhythm of life like the one we have nowadays - scattered along the way.