Self disclosure can be okay if it's in the interest of the client - but this is often done deliberately and carefully and the therapist will have considered whether it's helpful to share or not. Most of the time the therapist won't share personal information, and should be focused on you and your experience.
Knowing that you can say anything to your therapist and it will remain in the room helps you feel safe and builds trust between you and the therapist. For this reason, all therapists are legally and ethically bound to keep their sessions confidential and not share with anyone else what was talked about.
Boundary Issues: Self-disclosure can blur the professional boundaries between the counselor and the client. If a counselor shares too much personal information, it may lead to an inappropriate relationship or shift the focus away from the client's issues.