How do you know if someone is worth loving or not?

It is only worth loving a person if they repay the love you give them. One sided love is not love. A person who does not treat you well is not worth it. If they bring you happiness then they are worth loving. I think love should be mutual. If someone is constantly bringing you pain why should you love them? Do you love their actions or words? Those should determine if you actually love them or if their worth the hassle. People do change though and the new them might not be the old person you once loved.