The Thinking Brain (conscious thoughts, calculation, reasoning)
The Feeling Brain (desires, impulses, emotions).
As opposed to the classical assumption, it turns out our Feeling Brain sits on the driver seat. Thus, we are moved to action only by emotion. In the meantime, the Thinking Brain sits in the passenger seat and acts as the navigator.
Thinking Brain is the supporting character, but it imagines itself as the hero. It can't control the Feeling Brain, but can suggest shortcuts or more rational choices while taking action. Yet sometimes they are not the best ones.
That leads to the simplest and most obvious answer to the timeless question, why don't we do things we should do?