How can something be considered a loss when you never treated it as a gain?

In order to make a statement that something is yours, the car is yours, the house is yours, the relationship is yours, etc…, you have to support that claim. If someone says something doesn’t belong to you and they take you to court, you have to show proof it does. You have to be able to show in court you paid the mortgage to the house you say is yours, that you took care of that car you claim is yours, etc…. If someone else can prove they added value to something, they invested, they showed appreciation for it, the judge may rule in their favor, regardless of whether or not your name is on the dotted line. There are some women who under appreciate men and vice versa. How can something be considered a loss when you never treated it as a gain?