I think it’s because therapists are supposed to listen without judgment and in some cases they may offer or appear to offer “nurturing” that some people with mental health issues missed out on in childhood because of their parents’ difficulties or lack of caring.

So the client may feel attachment or even “romantic transference”.

Therapists aren’t supposed to dump their baggage (including with some sub optimal therapists lack of caring at times ) or anger etc on clients, but some do. Then “negative transference” may occur and retraumatisation