One reason that many people would agree with this is because everyone at one point in their life was nice, and kind hearted and filled with love but the people around them may crushed their spirits.
Another reason could be everyone has love in them no one is completely full of hate. They might have been hurt and hurt so many times but a little part of them is bright and full of hope.
People are not born mean and hateful they learn that as they grow up. They can learn in life that no one truly cares about them or they may think that. They may feel like they have no one and that this is a cruel world and they