I don't think there is anything wrong with positive thinking per se. But there is a difference between positive thinking and dreaming. Thinking positively can work if you understand you must have a strategy to attain what you ultimately want to attain. It doesn't just drop in your lap magically. Set goals and strategies to get there.
Also, understanding the amount of work it will take to attain what we want is also very important. Whatever we think it will take, multiply it by 10!