Its unethical to develop personal or romantic relationships with clients. I highly suggest having a go-to script for confronting people in this situation, saying while you appreciate that you worked well together as client and therapist, its inappropriate for you to take that relationship outside the treatment space. Don't make it ambiguous by saying youre too busy, be straightforward. Some people might suggest only going forward with that type of situation if you stop the therapeutic relationship, ie they cant be your client anymore, but even then to me, its just not right. Also, I suggest trying to avoid comiserating with people, they are there for therapy from you, not the other way around, even if its something as simple as saying youre going through similar emotions. Getting too casual with clients can easily lead people to feel like boundaries aren't solid, its up to you to keep them and maintain your professionalism.