I have asked my therapist what was going on and/or she has told me when she was a little "off." It's usually something like not sleeping well the night before. (Obviously if she were really unwell or extremely upset she would take the day off.) For me, it is sometimes about making sure I'm not doing something wrong or irritating her or causing her to push away, so it's useful information to have. This is something that comes up with other people in my life too, where they have been preoccupied or frustrated because of something else and I assume I'm the problem. So in that way it has been therapeutic for me to check in and then realize it's not me (and reflect on why I thought it was), although I do see where too much emphasis on the therapist's feelings or life stuff could be a very bad thing.