For example, we once had a client who had been convicted of a sexual offence against a child, and wanted access to his biological child. His attitude was poor and he lacked remorse. I found that very troubling, and it reduced me to tears at home thinking about it and left a "yucky" feeling, but that would not be evident to the client. Another reason I don't do family law either.
But, yeah, I imagine experienced criminal lawyers do detach to some extent and just get on with doing a good job.