In the first and second verses the Prophet teaches that happiness, as far as it is attainable in this world, is only to be had in conjunction with true justice. As the apostle teaches (Rom. 14), "For the kingdom of God is not meat and drink; but justice and peace and joy in the Holy Ghost." For the truly just are alone the friends of God, nay more, his children, and thus heirs of the kingdom, happy in the hope that belongs to the most perfect happiness, meanwhile, here below enjoying that solid joy and peace "that surpasseth all understanding." In this first verse he gives a negative description of the just man; in the second an affirmative, briefly stating here that he is just and thence happy who declines form evil and doth good.
[Saint Robert Bellarmine, A Commentary on the Book of Psalms, O'Sullivan, tr., Loreto Publicans (Fitzwilliam, NH: 2003), p. 1.]