What is "pleasant" in the sight of God and man is the union of two things, belief passing into profession, profession resting on belief.

For it is a pleasant thing if thou keep them within thee -
II. The pleasure and profit which may be derived from an attentive hearing.
1. They should be laid up in the heart-stored, treasured up within thee.
2. This will yield high satisfaction and happiness to the soul: "For it is a pleasant thing if thou keep them within thee."
3. The man who thus attends to the teachings of wisdom shall gain an experimental knowledge of them, so as to be able to speak of them suitably, pertinently and persuasively. "They shall withal be fitted in thy lips."

For it is a pleasant thing if thou keep them within thee,.... Or, "in thy belly" (a). That is, in thine heart, in the inmost recesses of it; where the words or doctrines of the wise should be received in the love of them, and carefully laid up and retained; which will upon reflection yield much pleasure, like Ezekiel's roll, which was in his belly as honey for sweetness; and which also is very profitable as an antidote against sin, Psalm 119:11;
they shall withal be fitted in thy lips; become them, and be suitable and graceful to them: or, "shall be ordered and disposed in" or "by thy lips" (b); being received into the heart, and digested there, they shall easily and freely go off the tongue, which shall be as the pen of a ready writer; they shall be delivered in a regular manner, with great liberty and facility; by a good digestion of Gospel truths, and a comfortable experience of them, persons become apt to teach others.
(a) "in ventre tuo", V. L. Montanus, Junius & Tremellius, Mercerus, Cocceius, Gejerus, Michaelis, Schultens. (b) "disponantur", Vatablus.

These lessons must be laid up in the mind, and
fitted--or better, "fixed" in the lips so as to be ever ready.

Fitted - Fitly expressed: thou wilt be able to discourse profitably of them.