Actually, transference doesn't only happen in therapy. It's a very common phenomenon but usually tends to be analyzed in the therapeutic relationship and not others. My perspective is that transference appears because we all have our own understanding of the world and patterns of relating. We do what we know in relationships, after trying to understand those around us usually based on previous experience. We often say about others that someone is like a mother figure or a big brother or a daughter ... Perhaps even more often, we make assumptions about people based on our experience with others rather than (just) moment-to-moment communication. I think that in a good therapeutic relationship, transference has the safe space to be explored and used in a healthy way, but transference happens whether or not there is a safe space. In my experience, even whether or not the therapist uses self-disclosure, because one can always find different ways to interpret and relate to the therapist's disclosure.