Nobel-prize winner Daniel Kahneman, along with his primary collaborator Amos Tversky, and many colleagues showed that we do not always reason "logically".
For example, a patient faced with a life and death decision is more likely to choose a treatment that has a 90% survival rate than to choose one that has a 10% fatality rate. But, of course, a treatment plan with a 90% survival rate has a 10% fatality rate: the two are identical. The only difference is in the framing: one is framed in terms of life (which is positive and desirable) the other in terms of death (undesirable).
In terms of motivating ourselves, and in terms of doing good work, I think that results of this sort emphasize the importance of choosing positive framings for how we see our project. The same project can be both boring (during much of the work) and exciting (at the moments when the work comes together and progress is made), etc. How we choose to frame it can affect our plans with respect to the work, and can affect our mood, and therefore our ability to work (at least to the extent that I believe that we work better when we're in a good mood).
We want to work on building a positive framing for how we see the project and for framing the project outcome, and let that serve as the primary focus. We want to be able to plan for the worst cases, and we want to have the ability to respond to unexpected obstacles--we don't, in short, want to be naive, imprudent or impetuous--but generally we want to focus our attention on what we are trying to create and how we are going to bring that into existence. We want to frame our analysis of our past behavior and results in terms of how we can learn to do better in the future. Such framings are more likely to contribute to active plans and less likely to create negative emotional drag.
There isn't just one way to look at the world. The same thing can be seen in different ways: the glass is half full and half empty. What the results of Kahneman, Tversky and their colleagues show is that the two different framings have an effect on plans (and on emotions, too, I speculate, though I don't know if that is a reported result). That is to say that if someone tells you "the glass is half full" you are likely to make a different decision than if someone tells you "the glass is half empty;" the fact that you make a different decision is indicative of a potential emotional effect of the framing.