Psalms 37:4 - Matthew Poole's English Annotations on the Holy Bible

Delight thyself in the Lord; in his favour and service, and in the study of his word and promises. The desires of thine heart, i.e. thy just desires, or whatsoever is truly desirable and good for thee; which limitation is necessarily to be understood, both from divers places of Scripture, and from the nature of the things; for it is unreasonable to imagine that God would engage himself to grant their sinful and inordinate desires, and it would also be a curse to them to have them granted.