Why can't the past stay there?

I can't help wondering why certain people can't let go of the people of their past. It isn't a bad thing unless the people in question are effecting your current everyday life. I try to get past it and not let it bother me, but I have found it impossible. There is a reason these people are in your past. Maybe you need to remind yourself why that is...get back to me when you figure it out.