We are human, we can not put hapiness on our face every time and also in my opinion positive thinking will destroy us sometimes.
Let me show you by taking an example.Suppose there is a man who have the degree of btech in his hand,somehow he got a job into a company similar to his study. He works hard there and also smartly, but he doesn't able to live his all dreams because the pay he is getting is enough not a large one.He is the man with hidden talent.Now suppose if he keep thinking positive i.e."A day will come when i will live all my dream, i will put more effort here" , then he is only lying with himself.He knows the company is paying him less, there is no fault in his work, then if he remain thinking positively, he is going to destroy himself.

So which thinking we should keep.Well my answer is constructive thinking.If the above man have constructive thinking, then he will think like this way" I am not able to live my dream life, isit fault in my work or in company". After couple of research he will find that the real fault was in company not in his, by this way he get out from the company and will start working there where his work get enough value.