You might have noticed while praying the psalter that sometimes the psalms jump from one topic to another in what seems to be a very random manner.
A psalm might start out with praise of God's glory. So you're obediently trying to put yourself in the attitude of rejoicing in creation, in God's power and love for his people,etc. Then without warning, the psalmhas launched into a diatribe against horrible enemies who are lying in wait to trap the righteous, and a prayer to God for protection from same. So again, you refocus, perhaps directing these lines to a plea on behalf of persecuted Christians, or against the assault made on the Church by a vicious media.
At which point, the psalm jumps back to joyous praise once more. Or thanksgiving. Or repentance. Who knows? Psalm 36 from today's Morning Prayer is a perfect example. It starts out with a discourse on the malice of sinners, but then leaps into a lyrical song about God's love. Then at the very end, there is one last swipe at the bad guys.
This can be disconcerting. We are prosaic people. We've been brought up on "transition sentences" in our essays that join a new idea to the old one. The psalms don't do this. Thomas Merton noticed this quirk in the psalms and explained it:
"For when one becomes conscious of who God really is,... the only possible reaction is the cry of half-articulate exultation that bursts from the depths of our being at the tremendous,inexplicable goodness of God to men. The psalms are made up of such cries--cries of wonder, exultation, anguish or joy. The very concreteness of their passion makes some of them seem disjointed and senseless. Their spontaneity makes them songs without plan, because there are no blueprints for ecstasy." (from Praying the Psalms)
This makes lots of sense of those psalms where the psalmist, after a long, despairing complaint about how bad life is, suddenly bursts into praise. Through all his pain, he still sees,with joy and wonder, the greatness of our God.
So now we see a reason for psalm whiplash. The Church has put some lovely aids in your breviary to help you see the main point or the "big picture" of the psalm despite the apparent randomness. These are the antiphons, the subtitle (written in red under the psalm title), and the scriptural or Church fathers' quotation (in black under the subtitle). All these are really, really helpful in seeing what the Church wants you to focus on as you pray each psalm or canticle. They make order of what sometimes appears to be prayerful chaos. The subtitle and quotation are optional. You can skip them when pressed for time, and I often do. But they are such excellent guides to the theme of each psalm that it's a good idea to check them out whenever you are not in a hurry. This morning, Psalm 36 was cleared up for me: the subtitle warned me that it would talk about both the malice of sinners as well as God's goodness. The quote from John's gospel, and the antiphon told me to focus on God as the Light of our lives.