I think everyone we meet in our life leaves a mark upon us in some way or other. We can never completely forget about people who have once crossed our lives somewhere in time. And it would be unjust to demand someone to completely forget those who have once been part of their lives. They were alive souls,they were a part of their lives,have affected their decisions,wishes and memories once in while and now here you are just demanding complete execution of their memories which is something nearly impossible. If you really want someone in your life, it is not through possessiveness that you are going to get him but through accepting him along with his past,the way he has been,the way he is,along with all of his memories and his loved ones; and there will be a time when those people will be a part of his unconsciousness but will never be forgotten. But lets look at the positive side of it,that someone you once had in your life and you do not want that person to forget you will always keep a part of you. And you may remain a part of their unconsciousness for a long time,but may be for once in their life-when they will be sitting along the shore on a bench or they will be in the middle of a dinner with their happy,ideal family or they will be in remorse and pain and there-you will cross their mind and that will not be a memory of unconsciousness but a conscious one a,and may be loking far into the sea,staring at the meal on their plate or breaking into pieces in that pain;your memory will leave a mark,they will know your worth in that moment. And that worth will be more valuable than the whole picture of a whole happy life with them. So,everyone you come across leaves a mark,even if the crossing is for a short while and sometimes short timed contacts,leave marks that are best of all marks.