What explains much of the pain you see on earth?

I am told by friends here that souls are sometimes reborn, reincarnated, to gain further experience, learn more life-lessons―or work out past sins and failings. Each earth life leaves its mark on character―its lessons are forever imprinted on the subconscious mind, which registers everything that has ever happened to the soul from the beginning.

This, they say, explains much of the pain and trouble you see on earth.