And delight thyself in Jehovah This delight is set in opposition to the vain and deceitful allurements of the world, which so intoxicate the ungodly, that despising the blessing of God, they dream of no other happiness than what presents itself for the time before their eyes. This contrast between the vain and fickle joys with which the world is deluded, and the true repose enjoyed by the godly, ought to be carefully observed; for whether all things smile upon us, or whether the Lord exercise us with adversities, we ought always to hold fast this principle, that as the Lord is the portion of our inheritance, our lot has fallen in pleasant places, as we have seen in Psalm 16:5,

Delight thyself also in the - Lord. The word rendered "delight" means properly to live delicately and effeminately; then, to be tender or delicate; then, to live a life of ease or pleasure; then, to find delight or pleasure in anything. The meaning here is, that we should seek our happiness in God - in his being, his perfections, his friendship, his love.
And he shall give thee the desires of thine heart - literally, the "askings," or the "requests" of thy heart. What you really "desire" will be granted to you. That is,
(a) the fact that you seek your happiness in him will regulate your desires, so that you will be "disposed" to ask only those things which it will be proper for him to grant; and
(b) the fact that you do find your happiness in him will be a reason why he will grant your desires.
The fact that a child loves his father, and finds his happiness in doing his will, will do much to regulate his own "wishes" or "desires," and will at the same thee be a reason why the father will be disposed to comply with his requests.

Delight thyself also in the Lord - Expect all thy happiness from him, and seek it in him.
The desires of thine heart - משאלות mishaloth, the petitions. The godly man never indulges a desire which he cannot form into a prayer to God.

Delight thyself also in the Lord,.... In the persons in God, Father, Son, and Spirit; in the perfections of God, his power, goodness, faithfulness, wisdom, love, grace, and mercy; in his works of creation, providence, and redemption; in his word, his Gospel, the truths and ordinances of it; in his house, and the worship of it; and in his people, the excellent in the earth, in whom was all the delight of the psalmist; and each of these afford a field of delight and pleasure, to attend unto, contemplate, and meditate upon;
and he shall give thee the desires of thine heart; such as are according to the will of God, and for the good of his people; such as relate to communion with him, and to the communication of more grace from him, and to the enjoyment of eternal glory.

desires-- (Psalm 20:5; Psalm 21:2), what is lawful and right, really good (Psalm 84:11).