This is actually very in line with what they are hypothesizing. Apparently a problem with positive fantasies is that they present an easy path to an idealized

This is actually very in line with what they are hypothesizing. Apparently a problem with positive fantasies is that they present an easy path to an idealized future situation. The path is missing any obstacles and possible hardships. And this somehow leads to an insufficient energy mobilization for pursuing that path. [makes sense from an planning/resource allocation perspective] As a "result" you don't put enough effort into tackling the encountering problems, once you reenter reality. The depressing effect comes later, when you face your low-achievement.