IMHO, being a good Uke (training partner) is very hard. You have to give just enough resistance so your partner can train effectively, but not so much that they can't train. Giving of yourself is sometime very hard.
When I am Tori/Nage, I always look at what I am doing wrong. When I am Uke, I alwasy look at what I am doing wrong to hinder my partners training. And I am wrong a lot.