4 He admonishes, moreover, by such an example, that no one ought to lift himself up in prosperous circumstances. For many fear adverse circumstances, fear not prosperous circumstances. Prosperity is more perilous to soul than adversity to body. First, prosperity does corrupt, in order that adversity may find something to break. My brethren, stricter watch must be kept against felicity. Wherefore, see ye after what manner the saying of God amid our own felicity does take from us security: “Serve ye,” He says, “the Lord in fear, and exult unto Him with trembling.” In exultation, in order that we may render thanks; in trembling, lest we fall.
This sin did not David, when he was suffering Saul for persecutor. When holy David was suffering Saul his enemy, when he was being vexed by his persecutions, when he was fleeing through various places, in order that he might not fall into his hands, he lusted not for her that was another's, he slew not husband after committing adultery with wife. He was in the infirmity of his tribulation so much the more intimate with God as he seemed more miserable. Something useful is tribulation; useful the surgeon's lancet rather than the devil's temptation.
He became secure when his enemies were overthrown, pressure was removed, swelling grew out. This example therefore does avail to this end, that we should fear felicity. “Tribulation,” he says, “and grief I found, and on the name of the Lord I called.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)