If you're assigning someone or something abilities they don't have, you're using "should" statements. For example, if you have a fear of flying and are telling yourself "I shouldn't be afraid of this, there's nothing wrong with the airplane!" you're putting an undue burden on yourself. You have a fear of flying! It's normal for people who have a fear of flying to be afraid flying!
Should statements can also be a sign of extreme unreasonable of yourself or others. You may think that you should be the best at your job, but this isn't necessarily true. Most people aren't the best; by definition, only one person can be! There's no universal requirement that it's got to be you.
If someone's rude to you, you may think:
They should have been more polite.
But why should they? Some people are not polite and it doesn't have to effect you.
I should be getting better faster.
They should have been here on time.