This is a distinction I was acutely experiencing as a teenager. As a child, I used to use fantasies to soothen my internal pain - from being mistreated (or "mistreated"), from being rejected and so on. It didn't occur to me until later that such refuge in imaginary ideas fails me: more obviously, just the fact that I'm thinking them doesn't make them any more true; less obviously, it encourages wishful and victim thinking both at the same time because I give away power and opportunity to change myself and my circumstances for the better since I'm too scared to act on them in reality. Those slowly went away as I started to work on myself. Positive thinking is helpful for one's mental health. Fantasizing is detrimental.
The article basically describes the planning fallacy. 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. The solution is simply to think rationally. While positive thinking is nice, in reality such is detrimental to achieving our goals. As for fantasizing, all things in moderation. Fantasizing allows us to see beyond the realm of possibility. At its best, it nurtures imagination and fosters hope. Allowing- just briefly- yourself to imagine what life could be, paired with rational goal setting, is what separates the day-to-day achievers from the entrepreneurs. After all, generations of humans had to fantasize over flight to one day invent the airplane.
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. 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.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.
As for fantasizing, all things in moderation.