1....Let this singer ascend; and let this man sing from the heart of each of you, and let each of you be this man, for when each of you says this, since you are all one in Christ, one man says this; and says not, “Unto You, O Lord, have” we “lift up” our “eyes;” but, “Unto You, O Lord, have I lift up my eyes”. You ought indeed to imagine that every one of you is speaking; but that One in a special sense speaks, who is also spread abroad over the whole world...
What makes the heart of a Christian heavy? Because he is a pilgrim, and longs for his country. If your heart be heavy on this score, although you have been prosperous in the world, still thou dost groan: and if all things combine to render you prosperous, and this world smile upon you on every side, thou nevertheless groanest, because you see that you are set in a pilgrimage; and feelest that you have indeed happiness in the eyes of fools, but not as yet after the promise of Christ: this you seek with groans, this you seek with longings, and by longing ascendest, and while you ascend dost sing the Song of Degrees.
2....Where then are the ladders? For we behold so great an interval between heaven and earth, there is so wide a separation, and so great a space of regions between: we wish to climb there, we see no ladder; do we deceive ourselves, because we sing the Song of Degrees, that is, the Song of ascent? We ascend unto heaven, if we think of God, who has made ascending steps in the heart. What is to ascend in heart? To advance towards God. As every man who fails, does not descend, but falls: so every one who profits does ascend: but if he so profit, as to avoid pride: if he so ascend as not to fall: but if while he profits he become proud, in ascending he again falls. But that he may not be proud, what ought he to do? Let him lift up his eyes unto Him who dwells in heaven, let him not heed himself...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)