Two people are involved, yes, but only one of those people is the focus of the relationship. In friendship, both people know a lot about the other, what they like, don't like, what their fears and vulnerabilities are. Part of friendship is taking into consideration the needs and feelings of the other person, based on what you know about them. Is your friend terrified of spiders? Don't talk about the wolf spider with all of the babies you found on the back porch that morning, etc.

In therapy, I don't know much about my therapist, so I don't have to take his fears and wants or opinions into consideration. It leaves me free to talk about my pet snakes even he's terrified of them, to talk about the way my mom screamed at me if his mom did the same and it is hard for him to hear. If I know those (totally hypothetical) things about him, I'd feel like I should hold back talking about them to protect him. At that point, it is not a therapeutic relationship anymore.

Does that make sense?