In truth, it's this kind of thinking that causes our suffering. Think about it; a different version of the past and/or present (how things "should have been" or how they "should be") exists only in our minds. Yet often these versions of reality cause a lot of suffering. When we set expectations for how things "should" be we become critical and judgmental when reality doesn't measure up. Our "shoulds" become absolute rules that govern every area of our lives. These rules are inflexible; there are no exceptions. This rigidity removes the option of acceptance when something doesn't go the way it "should." When you use the word "should" you implicitly make demands of both yourself and other people. While it might be reasonable to expect that you will meet your own demands, it is completely irrational to expect the rest of the world to do so.
The next time you find yourself thinking that something "should" or "should not" be the way it is, ask yourself this: Who says? Who gets to decide how things should or should not be? And who says that things should be any way other than exactly as they are? "Shoulds" are really just products of our minds. It is important to remember that just because our mind creates something does not mean it is true. In fact, frequently the products of our minds are actually the cause of great suffering.