The past can be hard for some people to remember. If someone were to bring up a painful past, let them know that you do not want to talk about it, and if they are persistent be persistent back until they drop the subject. There are some cases that the past is not painful, but something fun to remember. In case your past is something you do not mind talking about, share your side of the story or even ask the other person if they have any fun stories to share as well.
