There are times when we feel we are on top of the world. Every thing looks good and nothing is impossible. Similarly there are times when world looks gloomy. This is true with every individual.
When we are feeling high, it is important to keep the feet on the ground and remember that it is a transient phase. When we can do that, it helps us in dealing effectively those times when we are in lows.
It appears logical, but we tend to do the reverse. We fly when we are high and sink to the bottom during lows.
It is some time very difficult to come out of low feeling, and the negative thinking can put us in a vicious cycle of disaster.
Once the concept of "transient phase" is ingrained in our mind, we become more balanced and our behavior is appropriate. And our ability to pass these phases without making a fatal mistake improves many fold.
I really like this saying: "How you manage when you have nothing, and how you behave when you have everything - makes a man".