People imagine, and thus expect, the best possible circumstances and are consequently surprised when reality falls short. In that sense, positive thinking isn't always the best, but then neither is negative thinking.

This is where I must both elaborate on and edit what I said a bit. When I wrote the comment, I understood "positive thinking" to mean "optimism". I believe optimism to be the best option as far as planning for the future is concerned, because it reminds you that no matter how badly things go now, it will be okay later. It isn't to say that I believe in positive thinking making things okay, because the perception of them is all in our heads - so, by changing the way we think about things, we change the way we see them. Therefore, whether your plans fail, applying optimism means that you won't suffer because of the change.

As for fantasizing, all things in moderation.

Again, I must elaborate. By "fantasizing" here I understood "wishful thinking" - and the abuse of it for one's ego's sake, in particular. There's nothing wrong with imagination or using it: it's when you use it to bolster your self-image beyond what is real (that is, beyond what you actually are) that it becomes used badly. As I writer, I couldn't say otherwise; it was the terminology that I got confused.