2 “I will bless the Lord at all times; His praise shall be ever in my mouth”. So speaks Christ, so also let a Christian speak; for a Christian is in the Body of Christ; and therefore was Christ made Man, that that Christian might be enabled to be an Angel, who says, “I will bless the Lord at all times.” When shall I “bless the Lord”? When He blesses you? When the goods of this world abound? When you have great abundance of grain, oil, and wine, of gold and silver, of servants and cattle; when this mortal health remains unwounded and sound; when all that are born to you grow up, nothing is withdrawn by immature death, happiness wholly reigns in your house, and all things overflow around you; then shall you bless the Lord?
No; but “at all times.” Therefore both then, and when according to the time, or according to the scourges of our Lord God, these things are troubled, are taken away, are seldom born to you, and born pass away. For these things come to pass, and thence follows penury, need, labour, pain, and temptation. But you, who hast sung, “I will bless the Lord at all times: His praise shall be ever in my mouth,” both when He gives them, bless; and when He takes them away, bless. For it is He that gives, it is He that takes away: but Himself from him that blesses Him He takes not away.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)