Yesterday I talked about cost-benefit analysis and how it leads people to make decisions in life. It leads us to make all of our decisions. Well, it does when we are rational enough to think about it. This is a tool, and like any other tool, it is only useful when we think to use it.
Sometimes we just don't think. Sometimes we act instinctively or subconsciously. It could be that there is not enough time to make a rational decision, or it could be that a person was in such an emotional state that they were unable to take a step back and think things through.
Writing is a planned activity, and the results of it are a crafted work. Characters say the things that people wish they were clever enough to say off the cuff. Actions and reactions and interactions all nicely fall together to make an incredible set of circumstances. But real life is just not planned like that. Incredibly weird, coincidental things do occur, and people can be pretty witty at times, but we largely don't think things through or plan things well.
If you want to capture humanity, you have to see them throw cost-benefit analysis to the wind. They do something, not because they have determined it is the right thing to do, but because the basest instinct told them it was the thing to do.