Usually it's because that person has unfinished business within themselves. Something inside them wants to be forgiven for whatever mistake they made or to be given another chance. There's a level of cluelessness, an inability to work it out on their own and in desperation they reach out to that person. I had this experience with women friends that I had to let go. Years will pass and I will hear from them. They will ask me if I want to be friends again and I keep saying no. One friend I had to tell off really harshly and I never heard from her again. The others I thought I was pretty harsh with too but it seems they need to be completely cursed out and told off and I'm not willing to do alladat, raising my blood pressure. So I expect every year or two to get a text or something. Just something in them that can't move on. They realize after the fact how special the person was and are unable to replace them or find someone else like them.