I don't believe in coincidences the way most people do - not that they can't happen, but true coincidences, where there is absolutely no causal connection, are rarer than usually thought. It's like, say you get a feeling a friend emailed you, and you check your email, and hey, there's a message from her! But, you probably got that feeling other times when there wasn't an email from her, and you just forgot those. (Generic you.)

In your case, you probably get along with your therapist because you do have certain key similarities, and those are intellectual and personality-related, and you've been together a while (over a year?). So, yeah, it makes sense you're drawn to the same design, and you like an idea from his former therapist--which book you are likely drawn to because you're a trainee therapist yourself and your therapist has been influenced by this therapist. There are all kinds of ways these "coincidences" might never have surfaced - if you hadn't shared the book (which I think is the sort of thing he encourages you to do anyway), or if you'd liked the phone case design but went with another, cheaper one, or one your daughter liked, etc.

I would be bothered if he was scared by it, too. It suggests he might be reacting more emotionally than rationally, and you don't need that. But it also just might be an in-the-moment reaction. Either way, if you need to clear the air, clear the air.