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.
"Admit when you're wrong. Shut up when you're right." -John Gottman (This man knows a LOT about what helps - and what hurts - relationships. He's done tons of research on it, read his stuff it's amazing.
Only the FOOL reveals all that they know. The fool loves parading around with their seemingly infinite knowledge. They are always open to explaining EVERYTHING and ANYTHING. They tell their secrets habits tricks and even how much they have in their bank