NEGATIVE SIDE OF POSITIVE THINKING: Thoughts are neither negative nor positive. Thoughts just are. What they turn out to be depends on the context. Given a context, thoughts can be realistic or unrealistic, mean or generous, selfish or selfless. In reality, no positive thought can arise without a negative thought hiding around it. It is up to you to remain alert from negative side of positive thinking.
Positive thoughts can turn negative very quickly. Why? Because they organize in the same source. They are inter-connected. Negative thinking comes from the same-shared space as positive thinking. Thoughts become positive or negative depending on you. It is you who sets the standards and values by which you evaluate thoughts.
A lot of people talk about falling standards of every thing — food, music, newspapers, movies, the young generation, or whatever. Actually standards have neither fallen nor risen. Standards have just shifted. Your value frames have changed.