You're right, there is. Sometimes I see them as similar because students and teachers often have a similar type of dynamic. Students sometimes want to see their teacher break outside this role and appear more human. But once that happens the relationship isn't as effective. Therapy is, with all the intimacy, still a professional arrangement. If you're eating because you're starving and just can't wait until the end of session or couldn't fit it in beforehand then I can see a snack would be fine. But to use it as a way to create more intimacy or to give the feeling of having lunch with a friend, then it doesn't seem ideal.