I've only read the abstract, but here is my guess. There is a phenomenon where some people vividly fantasize about the "great results" of some yet-to-be-taken action or set of actions. The fantasizing itself brings them satisfaction. This satisfaction, then, in a sense undermines the sense of dissatisfaction that is necessary to motivation. Thus, the person never puts his plans into action. I would think that the resulting failure to achieve then triggers depression.

In plainer terms, for some people daydreaming is its own reward, but it's a short-term reward that undermines the effort needed to achieve reward in the long-term. When the dream fades and you're still where you are, that's depressing.