Why won't my boyfriend tell me what he talks to his therapist about?

My boyfriend of 6 months goes to a therapist every two weeks.

He says he just goes to talk, but he won't tell me what about. He says it's very helpful, he doesn't go because he has problems, he just considers it helpful.

Do I not have a right to know?

Plus, there should not be any secrets in a relationship.
Why won't my boyfriend tell me what he talks to his therapist about?
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