Unrealistic Expectations
In his post How to Make Behaviour Change Stick, Conor Neill explains that one reason why New Year's resolutions fail so frequently is that we start with unreasonable expectations. Unreasonable expectations, for example, about how quickly we will achieve our goals, about the effort it will take, or how quickly we will begin to see change. When things take longer or are harder than we expected, our commitment falters.
This doesn't apply only to personal resolutions. Daniel Kahneman, in his book Noise: A Flaw in Human Judgment, writes about what he calls base-rate neglect. For example, we estimate that a specific project in our company is going to take one month from start to finish, without realizing that similar projects have taken more than twice that time. Of course, the project doesn't get finished in a month. The error lies in making estimates from zero, without taking the outside view and considering how long projects like this usually take and how much effort they require. Again, unrealistic expectations.
When establishing ambitious goals, we need to dream big, and at the same time remain realistic about the effort it will take to achieve what we're aiming for. Otherwise, we'll get demoralized when things take longer or the road is not trouble-free as we incorrectly expected. As the saying goes, we tend to overestimate what we can do in a day but underestimate what we can achieve in a year.