In one breath David appeals to God, "Have mercy on me. Heal me, for I have sinned against you" (Psalm 41:4) Then in the next breath he claims to be innocent (v. 12). Perhaps David is not really sure about what he's praying. I've definitely heard people pray like that before, they go on and on, using lots of words but making little sense. David does, however, say something that makes a lot of sense, "Lord, have mercy on me. Make me well again, so I can pay them back!" (v. 10). This is in response to his enemies who gossip and whisper about him. It makes sense because I can understand wanting revenge. But there's something gross about it. Have mercy on me so I can get revenge. Really? Is that a plea God would/should listen to?
I'm enjoying the Psalms a lot less than I thought I would. Almost all of them sound the same- have mercy on me, save me from my enemies, crush my enemies, raise me up, preserve my life, blah…blah…blah. I've begun to almost just skim them because I'm completely unimpressed with the psalmist's, usually David's, obsessive self-centeredness and feeling of self-importance.
That's not to say there are some spectacular Psalms, they're just few are far between.