The Buddha spent his whole life trying to understand this one problem and find a solution to it: human suffering. And 2500 years later, with so many leaps and bounds of progressivism, we are pondering that exact same question (facepalm).
There is an internal experience and an external one. It's quite difficult to get at a person's true internal experience that leads them to the choices they make. I think it's reasonable that we haven't figured it out yet.
