From Feeling Good: The New Mood Therapy by David Burns
You try to motivate yourself by saying, "I should do this" or "I must do that." These statements cause you to feel pressured and resentful. Paradoxically, you end up feeling apathetic and unmotivated. Albert Ellis calls this "musturbation." I call it the "shouldy" approach to life.
When you direct should statements toward others, you will usually feel frustrated. When an emergency caused me to be five minutes late for the first therapy session, the new patient thought, "He shouldn't be so self-centered and thoughtless. He ought to be prompt." This thought caused her to feel sour and resentful.
Should statements generate a lot of unnecessary emotional turmoil in your daily life. When the reality of your own behavior falls short of your standards, your shoulds and shouldn'ts creat self-loathing, shame, and guilt. When the all-too-human performance of other people falls short of your expectations, as will inevitably happen from time to time, you'll feel bitter and self-righteous. You'll either have to change your expectations to approximate reality or always feel let down by human behavior. If you recognize this bad should habit in yourself, I have outlined many effective "should and shouldn't" removal methods in later chapters on guilt and anger.
This is a huge one for me, and I still struggle to shake it. Even right now as I type these posts out, I think "I should get off this computer and go to bed." But I don't do it. I just keep typing all the while feeling guilty because I'm not doing what I think I should. Telling myself I should do something doesn't motivate me to do it. I think it actually calls forth my rebellious side, which says, "Make me!"
Great, now I'm talking to myself talking to myself. Wheeeee!