You say that you are asking because you are curious and not judging but the questions posed are to my mind inherently judgemental. Are these people too dependent? Isn't it unhealthy. This is putting judgment on people's feelings and experiences - they are too dependent and it is unhealthy.

I find it more useful to ask why is it so - why is the person feeling the way they do? What need does this dependence communicate? What has happened to this person that has brought them into this moment where they have so strongly and intensely attached to their therapist? I do not like to think about as "too" something or along the dimension of healthy-unhealthy - it is what it is and it is this way for a good reason.

I also think that managing the dependence is the job of the therapist. Learning to tolerate the attachment feelings can be a very important therapeutic work for some patients. It is the job of the therapist to set boundaries that are not humiliating and overly punishing to the patient and that at the same time encourage the autonomous functioning of the patient and tolerating the attachment feelings without immediate reassurance, even if it happens very slowly over a long time.

So, in principle, I do not think that people themselves can be too attached or too reliant on their therapists but I do think that some(/many?) therapists might not know of have enough character or skill of how to properly work with such dependent and attachment feelings in order to promote discovery of authentic self and true autonomy (as opposed to just pushing the patient away, such that the patient feels deeply invalidated) while at the same time allowing and accepting all these feelings (as opposed to encouraging these feelings for the sake of some narcissistic feelings or saviour complex of whatever).