Sharing could help the person open up
Another quality I would say would be reliability. It could help if you went through a struggle of your own. It may not be the exact struggle - you don't have to be abused yourself; but it could be whether you were bullied, or whether you were made fun of, or whether you have low self-esteem. Any type of insecurity or sharing could help the other person to open up to you. Because, even though what you opened up about may not be insignificant, you are still confiding in them, and that could open the door for them to confide in you.