Perusing the General Instruction of the Liturgy of the Hours the other day, I came across this helpful passage. It answers two common questions.
First: What use is it for me to pray a happy psalm when I'm grieving or depressed, or a sad psalm when I'm feeling joyful?
Second: Isn't the Liturgy of the Hours only a "devotional" prayer (as opposed to a liturgical act) when prayed by a layperson, without the presence of a priest or religious mandated by the Church to pray the hours?
108. Those who pray the psalms in the liturgy of the hours do so not so much in their own name as in the name of the entire Body of Christ. This consideration does away with the problem of a possible discrepancy between personal feelings and the sentiments a psalm is expressing: for example, when a person feels sad and the psalm is one of joy or when a person feels happy and the psalm is one of mourning. Such a problem is readily solved in private prayer, which allows for the choice of a psalm suited to personal feelings. The divine office, however, is not private; the cycle of psalms is public, in the name of the Church, even for those who may be reciting an hour alone.[emphasis mine.-D.S.] Those who pray the psalms in the name of the Church nevertheless can always find a reason for joy or sadness, for the saying of the Apostle applies in this case also: "Rejoice with the joyful and weep with those who weep" (Rom 12:15). In this way human frailty, wounded by self-love, is healed in proportion to the love that makes the heart match the voice that prays the psalms.