Friends??

"I have never seen a man in civil life as nervous as Robert Cohn-nor as eager. I was envying it. It was lousy to enjoy it, but I felt lousy. Cohn had a wonderful quality of bringing out the worst in anybody." Page 104

I found this passage very interesting. In the first few pages of the novel, Jake states that he is one of Robert's two friends in Paris. I figured then that he obviously wasn't a popular guy. However, I assumed that at least Jake liked him. But in this passage, Jake is ammused by his naivete and enjoys his suffering. I don't usually do this to my friends. However, I will admit that Robert was getting extremely annoying.
His naivete intrigued me. How could this guy be so oblivious? Jake told him that Brett was going to marry Mike and Robert still acts like he has a chance. Why is he putting himself through this?