I think it also has to do with you coming off as a caretaker and a stabilizer. Borderlines want to find safety and stability and they want to be 'saved'. If you feel you have gotten some of the symptoms, I think it could be because you have witnessed people close to you push your boundaries and show you the depths of sadness which have affected you. Maybe you feel slightly broken too after having been pulled into a borderline's life and have had such an important emotional role to them, feeling you may be the source of what's wrong in their lives when they get upset.
I don't think you are borderline if you generally take the role of the caretaker, but I am no professional and if you think something may be amiss, I think you should ask someone who could give you the answers.