What are the negatives of 'always stay positive' approach?

It blinds one to the potential pitfalls & not make them learn life through failures, they grow only through their strengths & when they are unable to conceal their weaknesses and have to face them they fall flat on their head.

You might tend to not think through the complete picture of the scenario.

I believe in having a combo of Positivity and Negativity. Being positive makes you see the life in real perspective.at  the same time, it makes you to be aware of adversities in life. Where as over negativity makes you a cynic. It makes you a Pessimist

all in all, one should be able to separate the thorns and  enjoy the fragrance of Roses.  Utmost care has to be exercised to keep  the Negative attitudein its limits.

I don't see much negatives in a positive approach, it is the quality of being able to see the bright side which means you already know there is a dark side that needs to be overlooked.

Be positive attitude has got only one negative point. No one knows when the expected result will get fulfilled and while expecting something positive one needs to wait which in some cases may lead to an endless one. Time gets wasted but to get something one has to give up something precious. If luck favours then the time wasted in waiting becomes a worthwhile waste.

being positive in life definitely yeilds good result. However sometimes it is difficult to remain positive for a long time. We tend to turn to negative if things don't happen after being positive for a long time..

too much positive expectation from something that is not possible to get is not right from my point of view.

Such approach will make you unrealistic. You may become a day dreamer. 

the only disadvantage i can see of always being positive is that u may fail to feel or appreciate the happiness when u come out of a tough or bad situation. Quoting Emily Dickinson:

Success is counted sweetest
By those who never succeed.
To comprehend a nectar
Requires sorest need

Staying positive all the time is really advantageous as it helps you to sail through life easily, to see things in right perspective, to have patience for things to happen,, to give out positive vibrations where ever you go , to turn all experiences into a positive learning....and so on. And above all to stay healthy mentally and emotionally.

But coming back to it, we should not forget the saying "Excess of anything is bad" and it holds true in this scenario too. When you have excess of anything, it loses it's charm. If you are always positive, you fail to experience the negative emotions on the entire gamut of emotions that life offers. And without having a feel of those negative emotions, your acknowledgement to your real feeling is doubtful. You can cherish positive only when you have fought negatives and won over them. 

And I even feel that if you are always positive, you might miss out the real connect with people around you. People around you with be swinging between positive and negative and you might fail to acknowledge or even overlook when they are on a negative side. 

Constant positive Thinking can prevent us from thinking negative outcome and lead us to a state  of denial of our reality.