Gregory the Dialogist

Which rejoice exceedingly and are glad, when they can find the grave. For as the grave is that place wherein the body is buried, so heavenly contemplation is a kind of spiritual grave wherein the soul is buried. For in a certain sense we still live to this world, when in spirit we roam abroad therein. But we are buried in the grave as dead, when being mortified in things without, we secrete ourselves in the depths of interior contemplation. …

Morals on the Book of Job, Book V ↗

Thomas Aquinas

Because desire, when it is fulfilled causes joy, he adds, "and are glad powerfully when they find the grave," i.e. when they see they have arrived at death which procures a grave for them. Some think this passage refers to the fact that those who dig for treasure rejoice in finding a grave because they often found treasures in ancient tombs. But the first explanation is better.

Commentary on Job ↗