I've always had trouble seeing why this is termed irrational. Logically, if I'm the friend, I have a bargaining position and I can use it with the implicit threat of the other losing friendship, face, community standing, etc. The key thing is that social interaction doesn't happen in a vacuum. Not only will people reject what they consider a bad deal but, subject to similar experiments, so will most primates. This behavior evolved and was maintained through some millions of years of evolution. The only logical conclusion in my mind is that the way economics defines 'rational actors' is, itself, deeply irrational.