Because of this:
" For a therapist, it helps to be able to establish a sense of connection quickly with a wary client or gently disarm a hostile one. It’s great to have that just-right timing and sense of playfulness that gets a sullen teenager to crack a smile, or to say “mmm-hmm” with such a sense of deep rapport that it releases a tightly controlled client to finally let go and cry"

They are doing it at a client to get a reaction.