I read an interesting article today: ["Your app makes me fat"](http://seriouspony.com/blog/2013/7/24/your-app-makes-me-fat). Key quote:
"Researchers were astonished by a pile of experiments that led to one bizzare conclusion: Willpower and cognitive processing draw from the same pool of resources."
Now, when we tell people to behave rationally, we often tend to ask them to consider short term sacrifices for long term gains and act to maximise the overall "utility"; to run through a process of evaluation and taking action that uses up both cognitive processing and willpower at once.
I observed on many occasions that it is easy to make the "right' choice when you value the fact that you are trying to live your life in the right manner. The nice feels that you get when making the right choice compensate for the willpower expended in taking the corresponding actions.
And perhaps this is the value of "rationality" as a value.