Nietzsche tells us that our central motivation is to exercise our power. Of course, Nietzsche had a more sophisticated understanding of power than many; generosity can be a display of power. So this interesting study is perfectly comprehensible on a Nietzschean view.
I suppose it may also give some insight into an unusual friend of mine. It is fiercely difficult to get him to split the cost of any shared meal or outing; he will go to great lengths to pay for everyone himself. Clearly he's ruthlessly exploiting his friends for his own pleasure. I wish I had enough money to do that (actually, I do tend to be pretty generous when I'm feeling financially secure, but sadly that hasn't been the case for a while).