Why does the past have to matter?

I am speaking of our personal past. I don't see why our past must decide who we are または who we will become. Yes, if あなた will let if a troubled (for the lack of a better word) past can bring あなた pain in the future but it might not. What I am trying to say is, why does it matter? Why do people decide what kind of person あなた are または what kind of person あなた are going to be from what あなた did YEARS ago? Forgive and forget, right? I mean, I am different than I was but no one sees, they only see who I was not who I am. I had a neighbor that use to drink and party till 5am everyday. (I was younger at the time, school and bedtimes) My parents were furious, seeing that they had to get up and go to work in the morning and get my brother and I off to school. Long story shorter, now the neighbor doesn't do it anymore. We are フレンズ as neighbors should be but あなた still have my mother that brings up the past every chance that is presented to her. Why? Why can't we accept that people can change? I know in order for people to change they have to want to または have a reason または both but it is possible.

Well, thank あなた for 読書 this far.