Memories

What's never failed to make me question why is memories. And the reason I think why is because you can remember a certain thing happening, you can picture it in your head very easily and this memory can stay with you for a lifetime but then after a year or so you can't actually imagine yourself doing what you done in the memory, like it never happened.
I know what I'm saying is making no scene at all but just stick with me.
I'll give you an example, I used to be really close with somebody and I can remember so many things from the times when we were together but when I think back now it seems fake, like I've never done any if the things I can remember but of course I have because I wouldn't be able to remember it otherwise.
So my question is why does this happen?
Why do your memories seem like just a dream? I took some time to actually think about this and the result: your brain and body don't need you to think about the past, it doesn't matter to them any more, you've survived that time period and now there job is to ensure that you survive the present and the future because the past doesn't have that great of an impact to your life anymore so why focus on it when it can not be changed.