Attachment is natural with anyone you know/meet/work with. It's a continuum. Why or how you are attached makes a difference though. A therapeutic attachment you're probably more or less aware of and "using" to work through emotional attachments (with a parent or SO from the past). When you're a kid, you're chemically/naturally attached to your parents, especially your caretaker parent/mother and wholly helpless, etc., kind of imprinted and if it's not a good relationship, it causes emotional problems. If, in the relationship, even if it's a good one, you don't learn how to manage your emotions and use words effectively and "get along" with others, etc., if for whatever "reason" there's a problem while you're growing, that relationship develops a kink in it for you that could use working on later, in therapy.

The relationship developed in therapy is consciously developed by your therapist and is between two adults who didn't know each other before (so everything, by definition, has to be a reflection of the previous attachments) and it gets worked on in that therapeutic attachment. The attachment is not purely therapeutic though, you and your therapist do, naturally, care about/for one another! You couldn't work so intensively together without caring to that degree and having a caring, emotional bond! But it's more therapeutic (one knows it will deliberately end when therapy is done) than emotional as a parent/child or husband/wife, SO/SO relationship. It's not so much a joining of two as a leaning/supporting/learning of two like good therapy is. You know how two people can sit back-to-back and then both push and thus stand up together? I view therapy as kind of like that relationship.