Charles H. Spurgeon on Psalms 101:1

I will sing. The manner of expression imports a cordial resolution; heart and will are engaged in it; there is twice I will in the text. The manner of expression imports a humble resolution; I cannot sing of merit; but I will sing of mercy, and through mercy I will sing of mercy. To sing of mercy must be a humble song, for mercy towards a miserable sinner is a melting word; and to sing of judgment must be a humble song, for judgment in every sense is an awful word. The manner of the expression imports a skilful harper, a dexterous musician, even in a spiritual sense; he knew what should be the subject of the song, and he says, "I will sing of mercy and judgment"; and he knew what should be the object of the song, or to whom it should be sung, and therefore says, "To thee, O Lord, I will sing"; he knew who should be the singer, and therefore says, "I will" do it; he knew what should be the manner; and therefore says, "I will sing of mercy and judgment; to thee, O Lord, will I sing." It is before the Lord he resolves to sing, as he did before the ark, which was a type of Christ; and so is it s song to the praise of God in Christ. The manner of the expression imports a firm, fixed, and constant resolution; so the redoubling of it seems to import; "I will sing, I will sing." He had a mind this exercise of singing should not go down, but be his continual trade, "I will sing, I will sing"; I will sing on earth and I will sing in heaven; I will sing in time and I will sing in eternity. And, indeed, all on whom the spirit of praise and gratitude is poured out resolve never to give over singing... David had heard once, yea, twice, that mercy as well as power belongs to the Lord; and therefore not only once, but twice in a breath he resolves to sing unto the Lord. The word hath a great deal of elegancy and emphasis in it; I will sing of mercy, I will sing of judgment; O, I will sing, O Lord, I will sing; and I will sing unto thee. —Ralph Erskine.