To be fair, she didn’t really validate his feelings. It was more of a sneer and a scoff.
That is not validation, that’s condescension. If I had a therapist who looked at me this way or talked to me in the condescending tone she used with Dean, I’d run fast and I’d run far. And that’s not even touching the sketchy practice of pretending to be her patients’ dead loved ones. Her heart might have been in the right place with her patients (and with Jack because he knew what she was doing), but she isn’t a good therapist.
And I’d also like to point out, Dean said two episodes later that he wanted to die in part because he was just bringing his brother down. And yet no one has made the connection that Mia’s comments might have helped to solidify that impression?
Sam left because he was overwhelmed by his own grief for Mary, which he had/has yet to process, not because Dean upset him. She was right that he was directing his anger at the wrong place (not in this instance but in general and with Jack) but the way she went about saying so was not helpful in any way, shape or form. Of course it wasn’t all about her comments, but I’d say they certainly didn’t help. Just something to think about.
Your therapist isn’t supposed to make you feel guilty for things you do, they’re supposed to help you, whether it is to face them, cope with them, accept them or solve problems, therapists are there to help.