People look at the past through rose-tinted glasses, or whatever the term is. What it means is people tend to forget about the bad things.
Think back to when you were 8. Happy times, likely.
Everything was simpler, happier and just more vibrant, right?
When you were 8, I doubt you were thinking that.
When I think back to when I was 12 or 13, I was happy. I was content.
When I think about it hard enough, I start to remember that, at the time, it wasn't all that grand of a time for me.
Same thing.
My dad remembers his teen years as being the same thing. Fun, happy, exciting, adventurous, carefree...
In reality, the things that happened when he was a teen weren't very good things and he wasn't too happy of a person then.