A fool hath no delight in understanding. This may mean that he takes no pleasure in the wisdom of others, is self-opinionated; or, it may be, does not care for understanding in itself, apart from the use which he can make of it.
Vulgate, "The fool receives not the words of wisdom;" Septuagint, "A man of no sense has no need of wisdom." To try to teach a fool is to cast pearls before swine, and to give that which is holy unto dogs.
But that his heart may discover itself; i.e. his only delight is in revealing his heart, displaying his un-wisdom and his foolish thoughts, as in Proverbs 12:28; Proverbs 13:16; Proverbs 15:2. He thinks that thus he is showing himself superior to others, and benefiting the world at large.
The LXX. gives the reason, "For rather by folly he is led."