Because the smarter you are, the more you're aware of how little you really know. One of life's most important lessons is to take a moment before you speak, because what is very true for a lot of people is that the smarter you get the less you speak.
What is the difference between "I like you" and "I love you"? Beautifully answered by Buddha. When you like a flower, you just pluck it. But when you love a flower, you water it daily. One who understands this, understands life.