Adding to what everybody else said above, I suspect that for some therapists, we mean more to them and affect them more than we think we do (at least, those of us whose issues involve feeling insignificant, unworthy, etc). Jeffrey Kottler's "On Being a Therapist" talks about how the therapist can't help but be affected by his/her experiences with clients, and how it can lead to personal growth on the part of the therapist. There's a whole chapter devoted to the topic. Therapists keep boundaries to keep the environment safe and healthy for both people, but they are not hermetically sealed in a bubble.