13 But now that the man has found perfect happiness by keeping aloof from the counsel of the ungodly and the way of sinners and the seat of pestilence, and by gladly meditating in the Law of God by day and by night, we are next to be shown the rich fruit that this happiness he has won will yield him. Now the anticipation of happiness contains the germ of future happiness. For the next verse runs: And he shall be like a tree planted beside the rills of water, which shall yield its fruit in its own season, whose leaf also shall not fall off. This may perhaps be deemed an absurd and inappropriate comparison, in which are extolled a planted tree, rills of water, the yielding of fruit, its own time, and the leaf that falls not. All this may appear trivial enough to the judgment of the world. But let us examine the teaching of the Prophet and see the beauty that lies in the objects and words used to illustrate happiness.
Source: Homilies on the Psalms (New Advent)