I'm not surprised you didn't get it - i didn't make it as clear as I thought.
Ok, suppose you find yourself in a situation were you have various options what to do net.
Option 1 is evaluated by bb3 for its harm/benrfit to others and output a small amount of hormone accordingly.
The same option is also evaluated bty bb2 for its harm/benefit to yourself, also outputting hormone.
the outout of bb2 (self) signal may reinforce the bb3(others) signal, or it may be opposite in which case the stronger signal will 'win'. Hence an option very good for others and not very bad for self is felt as positive and vice versa.
by evaluating each option in turn and chosing the one that makes you happiest (ie produces the biggest 'dopamine minus seratonin' value) the decision what to do gets made.
the take-away is that you don't choose the platonic 'best' or 'most moral' option - you choose the one your brain's black boxes produces most 'dop minus ser' for. Natural selection hones that system to produce beneficial outcomes very much more often than not.