It’s not what happened, or who they were, or what they did.
You remember the best bits because you want to. You want to get to the good bits, you ignore the bad, you ignore the times when you felt rubbish.
You almost wonder, why did I cut them out of my life? You know you felt better when you did. You know you had to do what you had to do at that time.
You had to grow, and learn at that time. And to learn meant you had to recognise that not every friendship made you feel good.
You want an ideal, not the reality. You want a history, but not with the people who had that history with. You want something you cannot have.
So you idealise. You fantasise. You think it would have been better if you had kept them in your life.
But you know that wouldn’t be true. It wouldn’t.