Personal Biases
Many times, your own bias will keep you from seeing and hearing what others are saying. Of course, you can only see the world through your own filters. However, the more you know about your biases, the better you can put your ear to the ground and hear those you tend to dismiss or disagree with.
Some people let their personal difficulties interfere grossly with their work. During the course of life, one can expect some real joys and some real sorrows. Maintaining a proper perspective is often difficult. People may not reach peak performance because they avoid the really important challenges in life. Even some very intelligent people have very short attention spans.
Smart People
The next way smart people fail in life, because smart people are afraid of adapting. Let me explain why. When we have to change, it means that the existing us isn't good enough. And smart people don't like it to be known that they're not good enough. Smart people have been told they're smart all their lives. And now all of a sudden I'm telling you that you're not good enough and have to change? Yes, you're not good enough. You and I are not good enough to get to the next level we want in life. You have to adapt and change to be able to get there.